pretty: document multiple %(describe) being inconsistent

Each %(describe) placeholder is expanded using a separate git describe
call.  Their outputs depend on the tags present at the time, so there's
no consistency guarantee.  Document that fact.

Reported-by: Ævar Arnfjörð Bjarmason <[email protected]>
Signed-off-by: René Scharfe <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: rscharfe

Documentation/pretty-formats.txt

@@ -212,7 +212,9 @@ The placeholders are:
 			  linkgit:git-describe[1]; empty string for
 			  undescribable commits.  The `describe` string
 			  may be followed by a colon and zero or more
-			  comma-separated options.
+			  comma-separated options.  Descriptions can be
+			  inconsistent when tags are added or removed at
+			  the same time.
 +
 ** 'match=<pattern>': Only consider tags matching the given
    `glob(7)` pattern, excluding the "refs/tags/" prefix.