misc/vim: Document better syntax sync parameter

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

misc/vim/syntax/go.vim

@@ -198,6 +198,11 @@ endif
 hi def link     goExtraType         Type
 hi def link     goSpaceError        Error
 
+" Search backwards for a global declaration to start processing the syntax.
+"syn sync match goSync grouphere NONE /^\(const\|var\|type\|func\)\>/
+
+" There's a bug in the implementation of grouphere. For now, use the
+" following as a more expensive/less precise workaround.
 syn sync minlines=500
 
 let b:current_syntax = "go"