go/build: clarify that there are no build tags for minor releases

Fixes #26458

Change-Id: If932718ca8a2b230ab52495c1a7a82d86ab1325b
Reviewed-on: https://go-review.googlesource.com/136215Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/go/build/doc.go

@@ -110,6 +110,9 @@
 //	- "go1.11", from Go version 1.11 onward
 //	- any additional words listed in ctxt.BuildTags
 //
+// There are no build tags for beta or minor releases. Programs that need the
+// minor release number can call runtime.Version.
+//
 // If a file's name, after stripping the extension and a possible _test suffix,
 // matches any of the following patterns:
 //	*_GOOS