cmd/go/internal/{run,work,generate}: document file path location requirement

Mention that .go files must be from a single directory
for them to be treated as a single package.

Fixes #21529

Change-Id: I79cb08b9f43888814b1249a7b50bc7bc70bc1c72
Reviewed-on: https://go-review.googlesource.com/c/go/+/149797


Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>