gc doc: mention the commit-graph in the intro

Explicitly mention in the intro that we may be writing supplemental
data structures such as the commit-graph during "gc", i.e. to call out
the "optimize" part of what this command does, it doesn't just
"collect garbage" as the "gc" name might imply.

Past changes have updated the intro to reflect new commands, such as
mentioning "worktree" in b586a96a39 ("gc.txt: more details about what
gc does", 2018-03-15). So let's elaborate on what was added in
d5d5d7b641 ("gc: automatically write commit-graph files", 2018-06-27).

See also
https://public-inbox.org/git/87tvm3go42.fsf@evledraar.gmail.com/ (follow-up
replies) for an on-list discussion about what "gc" does.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-gc.txt

@@ -17,7 +17,8 @@ Runs a number of housekeeping tasks within the current repository,
 such as compressing file revisions (to reduce disk space and increase
 performance), removing unreachable objects which may have been
 created from prior invocations of 'git add', packing refs, pruning
-reflog, rerere metadata or stale working trees.
+reflog, rerere metadata or stale working trees. May also update ancillary
+indexes such as the commit-graph.
 
 Users are encouraged to run this task on a regular basis within
 each repository to maintain good disk space utilization and good