go/build: document characters allowed in build tags

The documentation just said "alphanumeric", but underscores and dots
are also accepted.

Fixes #32886

Change-Id: I1ba872a220d5c5bf64f1d851ddba9eb3b1afb89a
Reviewed-on: https://go-review.googlesource.com/c/go/+/184917


Reviewed-by: Bryan C. Mills <bcmills@google.com>