Forcing the synchronization of 500 lines is both slower and
less precise than searching for a known sync point.
Unfortunately, though, the way to synchronize correctly is
using the grouphere instruction, which has a bug.

I've already proposed the fix to Vim, so future releases
should have this working.  We can continue using the 500 lines
syncing for now.

This change just documents the proper way to sync.

R=adg, dsymonds
CC=golang-dev
https://golang.org/cl/4029047