go/build: mention 'tag' as an alias for 'build constraint'

The code is all about tags, and the cmd/go documentation
said to look in the go/build documentation for information
about tags, but the documentation said nothing about tags,
only build constraints. Make things clearer.

R=golang-dev, adg, rsc
CC=golang-dev
https://golang.org/cl/44100043

src/cmd/go/build.go

@@ -87,8 +87,8 @@ The build flags are shared by the build, install, run, and test commands:
 		arguments to pass on each 5l, 6l, or 8l linker invocation.
 	-tags 'tag list'
 		a list of build tags to consider satisfied during the build.
-		See the documentation for the go/build package for
-		more information about build tags.
+		For more information about build tags, see the description of
+		build constraints in the documentation for the go/build package.
 
 The list flags accept a space-separated list of strings. To embed spaces
 in an element in the list, surround it with either single or double quotes.

src/cmd/go/doc.go

@@ -104,8 +104,8 @@ The build flags are shared by the build, install, run, and test commands:
 		arguments to pass on each 5l, 6l, or 8l linker invocation.
 	-tags 'tag list'
 		a list of build tags to consider satisfied during the build.
-		See the documentation for the go/build package for
-		more information about build tags.
+		For more information about build tags, see the description of
+		build constraints in the documentation for the go/build package.
 
 The list flags accept a space-separated list of strings. To embed spaces
 in an element in the list, surround it with either single or double quotes.

src/pkg/go/build/doc.go

@@ -57,11 +57,15 @@
 //
 // Build Constraints
 //
-// A build constraint is a line comment beginning with the directive +build
+// A build constraint, also known as a build tag, is a line comment that begins
+//
+//	// +build
+//
 // that lists the conditions under which a file should be included in the package.
 // Constraints may appear in any kind of source file (not just Go), but
 // they must appear near the top of the file, preceded
-// only by blank lines and other line comments.
+// only by blank lines and other line comments. These rules mean that in Go
+// files a build constraint must appear before the package clause.
 //
 // To distinguish build constraints from package documentation, a series of
 // build constraints must be followed by a blank line.