docs: document --textconv diff option

This has been there since textconv existed, but was never
documented. There is some overlap with what's in
gitattributes(5), but it's important to warn in both places
that textconv diffs probably can't be applied.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/diff-options.txt

@@ -416,6 +416,17 @@ endif::git-format-patch[]
 --no-ext-diff::
 	Disallow external diff drivers.
 
+--textconv::
+--no-textconv::
+	Allow (or disallow) external text conversion filters to be run
+	when comparing binary files. See linkgit:gitattributes[5] for
+	details. Because textconv filters are typically a one-way
+	conversion, the resulting diff is suitable for human
+	consumption, but cannot be applied. For this reason, textconv
+	filters are enabled by default only for linkgit:git-diff[1] and
+	linkgit:git-log[1], but not for linkgit:git-format-patch[1] or
+	diff plumbing commands.
+
 --ignore-submodules[=<when>]::
 	Ignore changes to submodules in the diff generation. <when> can be
 	either "none", "untracked", "dirty" or "all", which is the default