doc: add more pointers to gitattributes(5) for userdiff

Several Git commands can make use of the builtin userdiff patterns, but
it's not obvious in the documentation. Add pointers to the 'Defining a
custom hunk header' part of gitattributes(5) in the description of the
following options:

- the '--function-context' option of `git diff` and friends
- the '--function-context' option of `git grep`
- the '-L :<funcname>' option of `git log`, `gitk` and `git blame`

In 'git-grep.txt', take the opportunity to use backticks in the
description of '--show-function', and improve the wording of the
desription of '--function-context'.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/diff-options.txt

@@ -695,7 +695,10 @@ endif::git-format-patch[]
 
 -W::
 --function-context::
-	Show whole surrounding functions of changes.
+	Show whole function as context lines for each change.
+	The function names are determined in the same way as
+	`git diff` works out patch hunk headers (see 'Defining a
+	custom hunk-header' in linkgit:gitattributes[5]).
 
 ifndef::git-format-patch[]
 ifndef::git-log[]

Documentation/git-grep.txt

@@ -241,7 +241,7 @@ providing this option will cause it to die.
 --show-function::
 	Show the preceding line that contains the function name of
 	the match, unless the matching line is a function name itself.
-	The name is determined in the same way as 'git diff' works out
+	The name is determined in the same way as `git diff` works out
 	patch hunk headers (see 'Defining a custom hunk-header' in
 	linkgit:gitattributes[5]).
 
@@ -266,7 +266,9 @@ providing this option will cause it to die.
 	Show the surrounding text from the previous line containing a
 	function name up to the one before the next function name,
 	effectively showing the whole function in which the match was
-	found.
+	found. The function names are determined in the same way as
+	`git diff` works out patch hunk headers (see 'Defining a
+	custom hunk-header' in linkgit:gitattributes[5]).
 
 --threads <num>::
 	Number of grep worker threads to use.

Documentation/line-range-format.txt

@@ -27,4 +27,6 @@ regular expression that denotes the range from the first funcname line
 that matches '<funcname>', up to the next funcname line. `:<funcname>`
 searches from the end of the previous `-L` range, if any, otherwise
 from the start of file. `^:<funcname>` searches from the start of
-file.
+file. The function names are determined in the same way as `git diff`
+works out patch hunk headers (see 'Defining a custom hunk-header'
+in linkgit:gitattributes[5]).