cmd/go: clarify relative import paths and modules in documentation

[perillo]

In the documentation for cmd/go, at the end of the section about relative import paths
https://golang.org/cmd/go/#hdr-Remote_import_paths, the text says:

To avoid ambiguity, Go programs cannot use relative import paths within a work space.

The documentation should also add that relative import paths don't work with modules.

[gopherbot]

Change https://golang.org/cl/178120 mentions this issue: doc: Clarify relative import paths don't work with modules.