doc: document the lack of support for symlinks under GOPATH

In an effort to help others avoid the issues I've hit due to lack of
symlink support under GOPATH, I've added a note of warning to the
Workspaces section.

I have not changed the contents of go help gopath, because on reflection
it seems this change alone may be sufficient.

Fixes #21320

Change-Id: Ib8969bf12cecad878e89ff66b5864bbf3caaf219
Reviewed-on: https://go-review.googlesource.com/61930


Reviewed-by: Ian Lance Taylor <iant@golang.org>