Documentation/git-multi-pack-index.txt: clarify expire behavior

The `expire` sub-command of `git multi-pack-index` will never expire
`.keep` packs, regardless of whether or not any of their objects were
selected in the MIDX.

This has always been the case since 19575c7 (multi-pack-index:
implement 'expire' subcommand, 2019-06-10), which came after cff9711
(multi-pack-index: prepare for 'expire' subcommand, 2019-06-10), when
this documentation was originally written.

Signed-off-by: Taylor Blau <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: ttaylorr

Documentation/git-multi-pack-index.txt

@@ -71,8 +71,9 @@ verify::
 
 expire::
 	Delete the pack-files that are tracked by the MIDX file, but
-	have no objects referenced by the MIDX. Rewrite the MIDX file
-	afterward to remove all references to these pack-files.
+	have no objects referenced by the MIDX (with the exception of
+	`.keep` packs). Rewrite the MIDX file afterward to remove all
+	references to these pack-files.
 
 repack::
 	Create a new pack-file containing objects in small pack-files