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>