cmd/go: change +build doc references to //go:build

This changes a few references to `+build` into the modern `//go:build`.
It was compiled by editing `cmd/go/internal/list/context.go`, running
`go test cmd/go -v -run=TestDocsUpToDate -fixdocs`, and then editing
list.go and build.go by hand.

src/cmd/go/alldocs.go

@@ -883,9 +883,9 @@
 //	    GOROOT        string   // Go root
 //	    GOPATH        string   // Go path
 //	    CgoEnabled    bool     // whether cgo can be used
-//	    UseAllFiles   bool     // use files regardless of +build lines, file names
+//	    UseAllFiles   bool     // use files regardless of //go:build lines, file names
 //	    Compiler      string   // compiler to assume when computing target paths
-//	    BuildTags     []string // build constraints to match in +build lines
+//	    BuildTags     []string // build constraints to match in //go:build lines
 //	    ToolTags      []string // toolchain-specific build constraints
 //	    ReleaseTags   []string // releases the current release is compatible with
 //	    InstallSuffix string   // suffix to use in the name of the install dir

src/cmd/go/internal/list/context.go

@@ -14,7 +14,7 @@ type Context struct {
 	GOROOT        string   `json:",omitempty"` // Go root
 	GOPATH        string   `json:",omitempty"` // Go path
 	CgoEnabled    bool     `json:",omitempty"` // whether cgo can be used
-	UseAllFiles   bool     `json:",omitempty"` // use files regardless of +build lines, file names
+	UseAllFiles   bool     `json:",omitempty"` // use files regardless of //go:build lines, file names
 	Compiler      string   `json:",omitempty"` // compiler to assume when computing target paths
 	BuildTags     []string `json:",omitempty"` // build constraints to match in +build lines
 	ToolTags      []string `json:",omitempty"` // toolchain-specific build constraints

src/cmd/go/internal/list/list.go

@@ -148,9 +148,9 @@ The template function "context" returns the build context, defined as:
         GOROOT        string   // Go root
         GOPATH        string   // Go path
         CgoEnabled    bool     // whether cgo can be used
-        UseAllFiles   bool     // use files regardless of +build lines, file names
+        UseAllFiles   bool     // use files regardless of //go:build lines, file names
         Compiler      string   // compiler to assume when computing target paths
-        BuildTags     []string // build constraints to match in +build lines
+        BuildTags     []string // build constraints to match in //go:build lines
         ToolTags      []string // toolchain-specific build constraints
         ReleaseTags   []string // releases the current release is compatible with
         InstallSuffix string   // suffix to use in the name of the install dir

src/cmd/go/internal/modindex/build.go

@@ -42,7 +42,7 @@ type Context struct {
 	Dir string
 
 	CgoEnabled  bool   // whether cgo files are included
-	UseAllFiles bool   // use files regardless of +build lines, file names
+	UseAllFiles bool   // use files regardless of //go:build lines, file names
 	Compiler    string // compiler to assume when computing target paths
 
 	// The build, tool, and release tags specify build constraints