Add missing documentation of --current-branch option on sync command

Change-Id: I72d6e3d51241148c1df97bbad26338debb1fcb4e

subcmds/sync.py

@@ -128,6 +128,9 @@ HTTP client or proxy configuration, but the Git binary works.
 The --fetch-submodules option enables fetching Git submodules
 of a project from server.
 
+The -c/--current-branch option can be used to only fetch objects that
+are on the branch specified by a project's revision.
+
 SSH Connections
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