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>

Author: ncantelmo

doc/code.html

@@ -110,6 +110,10 @@ <h3 id="Workspaces">Workspaces</h3>
 and dependencies in a single workspace.
 </p>
 
+<p>
+Note that symbolic links should <b>not</b> be used to link files or directories into your workspace.
+</p>
+
 <p>
 Commands and libraries are built from different kinds of source packages.
 We will discuss the distinction <a href="#PackageNames">later</a>.