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>