diff format documentation: clarify --cc and -c

The description was unclear if -c or --cc was the default (--cc is for
some commands), and incorrectly implied that the default applies to
all the diff generating commands.

Most importantly, "log" does not default to "--cc" (it defaults to
"--no-merges") and "log -p" obeys the user's wish to see non-combined
format.  Only "diff" (during merge and three-blob comparison) and
"show" use --cc as the default.
Signed-off-by: NAdam Monsen <haircut@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/diff-generate-patch.txt

@@ -74,10 +74,13 @@ separate lines indicate the old and the new mode.
 combined diff format
 --------------------
 
-"git-diff-tree", "git-diff-files" and "git-diff" can take '-c' or
-'--cc' option to produce 'combined diff'.  For showing a merge commit
-with "git log -p", this is the default format; you can force showing
-full diff with the '-m' option.
+Any diff-generating command can take the `-c` or `--cc` option to
+produce a 'combined diff' when showing a merge. This is the default
+format when showing merges with linkgit:git-diff[1] or
+linkgit:git-show[1]. Note also that you can give the `-m' option to any
+of these commands to force generation of diffs with individual parents
+of a merge.
+
 A 'combined diff' format looks like this:
 
 ------------