docs/diff-options: clarify scope of diff-filter types

The same document for "--diff-filter" is included by many
programs in the diff family. Because it mentions all
possible types (added, removed, etc), this may imply to the
reader that all types can be generated by a particular
command. But this isn't necessarily the case; "diff-files"
cannot generally produce an "Added" entry, since the diff is
limited to what is already in the index.

Let's make it clear that the list here is the full one, and
does not imply anything about what a particular invocation
may produce.

Note that conditionally including items (e.g., omitting
"Added" in the git-diff-files manpage) isn't the right
solution here for two reasons:

  - The problem isn't diff-files, but doing an index to
    working tree diff. "git diff" can do the same diff, but
    also has other modes where "Added" does show up.

  - The direction of the diff matters. Doing "diff-files -R"
    can get you Added entries (but not Deleted ones).

So it's best just to explain that the set of available types
depends on the specific diff invocation.

Reported-by: John Cheng <johnlicheng@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/diff-options.txt

@@ -422,6 +422,12 @@ ifndef::git-format-patch[]
 +
 Also, these upper-case letters can be downcased to exclude.  E.g.
 `--diff-filter=ad` excludes added and deleted paths.
++
+Note that not all diffs can feature all types. For instance, diffs
+from the index to the working tree can never have Added entries
+(because the set of paths included in the diff is limited by what is in
+the index).  Similarly, copied and renamed entries cannot appear if
+detection for those types is disabled.
 
 -S<string>::
 	Look for differences that change the number of occurrences of