update: make --help consistent, add options

Document short options and common global options. Apply the same indent
as other command help strings.

Library/Homebrew/cmd/update.sh

@@ -2,8 +2,11 @@
 #:
 #:  Fetch the newest version of Homebrew and all formulae from GitHub using `git`(1) and perform any necessary migrations.
 #:
-#:       --merge                         `git merge` is used to include updates (rather than `git rebase`).
-#:       --force                         Always do a slower, full update check (even if unnecessary).
+#:          --merge                      `git merge` is used to include updates (rather than `git rebase`).
+#:      -f, --force                      Always do a slower, full update check (even if unnecessary).
+#:      -v, --verbose                    Print the directories checked and `git` operations performed.
+#:      -d, --debug                      Display a trace of all shell commands as they are executed.
+#:      -h, --help                       Show this message.
 
 # Don't need shellcheck to follow this `source`.
 # shellcheck disable=SC1090