Documentation: add extensions.worktreeConfig details

The extensions.worktreeConfig extension was added in 58b284a (worktree:
add per-worktree config files, 2018-10-21) and was somewhat documented
in Documentation/git-config.txt. However, the extensions.worktreeConfig
value was not specified further in the list of possible config keys. The
location of the config.worktree file is not specified, and there are
some precautions that should be mentioned clearly, but are only
mentioned in git-worktree.txt.

Expand the documentation to help users discover the complexities of
extensions.worktreeConfig by adding details and cross links in these
locations (relative to Documentation/):

- config/extensions.txt
- git-config.txt
- git-worktree.txt

The updates focus on items such as

* $GIT_DIR/config.worktree takes precedence over $GIT_COMMON_DIR/config.

* The core.worktree and core.bare=true settings are incorrect to have in
  the common config file when extensions.worktreeConfig is enabled.

* The sparse-checkout settings core.sparseCheckout[Cone] are recommended
  to be set in the worktree config.

As documented in 11664196ac ("Revert "check_repository_format_gently():
refuse extensions for old repositories"", 2020-07-15), this extension
must be considered regardless of the repository format version for
historical reasons.

A future change will update references to extensions.worktreeConfig
within git-sparse-checkout.txt, but a behavior change is needed before
making those updates.

Helped-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config/extensions.txt

@@ -6,3 +6,34 @@ extensions.objectFormat::
 Note that this setting should only be set by linkgit:git-init[1] or
 linkgit:git-clone[1].  Trying to change it after initialization will not
 work and will produce hard-to-diagnose issues.
+
+extensions.worktreeConfig::
+	If enabled, then worktrees will load config settings from the
+	`$GIT_DIR/config.worktree` file in addition to the
+	`$GIT_COMMON_DIR/config` file. Note that `$GIT_COMMON_DIR` and
+	`$GIT_DIR` are the same for the main working tree, while other
+	working trees have `$GIT_DIR` equal to
+	`$GIT_COMMON_DIR/worktrees/<id>/`. The settings in the
+	`config.worktree` file will override settings from any other
+	config files.
++
+When enabling `extensions.worktreeConfig`, you must be careful to move
+certain values from the common config file to the main working tree's
+`config.worktree` file, if present:
++
+* `core.worktree` must be moved from `$GIT_COMMON_DIR/config` to
+  `$GIT_COMMON_DIR/config.worktree`.
+* If `core.bare` is true, then it must be moved from `$GIT_COMMON_DIR/config`
+  to `$GIT_COMMON_DIR/config.worktree`.
++
+It may also be beneficial to adjust the locations of `core.sparseCheckout`
+and `core.sparseCheckoutCone` depending on your desire for customizable
+sparse-checkout settings for each worktree. By default, the `git
+sparse-checkout` builtin enables `extensions.worktreeConfig`, assigns
+these config values on a per-worktree basis, and uses the
+`$GIT_DIR/info/sparse-checkout` file to specify the sparsity for each
+worktree independently. See linkgit:git-sparse-checkout[1] for more
+details.
++
+For historical reasons, `extensions.worktreeConfig` is respected
+regardless of the `core.repositoryFormatVersion` setting.

Documentation/git-config.txt

@@ -141,9 +141,13 @@ from all available files.
 See also <<FILES>>.
 
 --worktree::
-	Similar to `--local` except that `.git/config.worktree` is
+	Similar to `--local` except that `$GIT_DIR/config.worktree` is
 	read from or written to if `extensions.worktreeConfig` is
-	present. If not it's the same as `--local`.
+	enabled. If not it's the same as `--local`. Note that `$GIT_DIR`
+	is equal to `$GIT_COMMON_DIR` for the main working tree, but is of
+	the form `$GIT_DIR/worktrees/<id>/` for other working trees. See
+	linkgit:git-worktree[1] to learn how to enable
+	`extensions.worktreeConfig`.
 
 -f <config-file>::
 --file <config-file>::

Documentation/git-worktree.txt

@@ -286,8 +286,8 @@ CONFIGURATION FILE
 ------------------
 By default, the repository `config` file is shared across all working
 trees. If the config variables `core.bare` or `core.worktree` are
-already present in the config file, they will be applied to the main
-working trees only.
+present in the common config file and `extensions.worktreeConfig` is
+disabled, then they will be applied to the main working tree only.
 
 In order to have configuration specific to working trees, you can turn
 on the `worktreeConfig` extension, e.g.:
@@ -307,11 +307,16 @@ them to the `config.worktree` of the main working tree. You may also
 take this opportunity to review and move other configuration that you
 do not want to share to all working trees:
 
- - `core.worktree` and `core.bare` should never be shared
+ - `core.worktree` should never be shared.
+
+ - `core.bare` should not be shared if the value is `core.bare=true`.
 
  - `core.sparseCheckout` is recommended per working tree, unless you
    are sure you always use sparse checkout for all working trees.
 
+See the documentation of `extensions.worktreeConfig` in
+linkgit:git-config[1] for more details.
+
 DETAILS
 -------
 Each linked working tree has a private sub-directory in the repository's