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/+/184917Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/go/build/doc.go

@@ -70,10 +70,11 @@
 // To distinguish build constraints from package documentation, a series of
 // build constraints must be followed by a blank line.
 //
-// A build constraint is evaluated as the OR of space-separated options;
-// each option evaluates as the AND of its comma-separated terms;
-// and each term is an alphanumeric word or, preceded by !, its negation.
-// That is, the build constraint:
+// A build constraint is evaluated as the OR of space-separated options.
+// Each option evaluates as the AND of its comma-separated terms.
+// Each term consists of letters, digits, underscores, and dots.
+// A term may be negated with a preceding !.
+// For example, the build constraint:
 //
 //	// +build linux,386 darwin,!cgo
 //