cmd/go: add documentation for test and xtest fields output by go list

The TestEmbedPatterns, TestEmbedFiles, XTestEmbedPatterns, and
XTestEmbedFiles fields were left out of golang.org/cl/282195 which was
supposed to document the embed fields available in the go list
output. Add documentation for them in this CL.

Fixes #43081

Change-Id: Ifc256c476daec7c0f0e2c41f86b82f958b3e2b1a
Reviewed-on: https://go-review.googlesource.com/c/go/+/284258
Trust: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

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

@@ -90,8 +90,12 @@ to -f '{{.ImportPath}}'. The struct being passed to the template is:
         XTestGoFiles    []string   // _test.go files outside package
 
         // Embedded files
-        EmbedPatterns []string // //go:embed patterns
-        EmbedFiles    []string // files and directories matched by EmbedPatterns
+        EmbedPatterns      []string // //go:embed patterns
+        EmbedFiles         []string // files matched by EmbedPatterns
+        TestEmbedPatterns  []string // //go:embed patterns in TestGoFiles
+        TestEmbedFiles     []string // files matched by TestEmbedPatterns
+        XTestEmbedPatterns []string // //go:embed patterns in XTestGoFiles
+        XTestEmbedFiles    []string // files matched by XTestEmbedPatterns
 
         // Cgo directives
         CgoCFLAGS    []string // cgo: flags for C compiler

src/cmd/go/internal/load/pkg.go

@@ -96,7 +96,7 @@ type PackagePublic struct {
 
 	// Embedded files
 	EmbedPatterns []string `json:",omitempty"` // //go:embed patterns
-	EmbedFiles    []string `json:",omitempty"` // files and directories matched by EmbedPatterns
+	EmbedFiles    []string `json:",omitempty"` // files matched by EmbedPatterns
 
 	// Cgo directives
 	CgoCFLAGS    []string `json:",omitempty"` // cgo: flags for C compiler
@@ -122,11 +122,11 @@ type PackagePublic struct {
 	TestGoFiles        []string `json:",omitempty"` // _test.go files in package
 	TestImports        []string `json:",omitempty"` // imports from TestGoFiles
 	TestEmbedPatterns  []string `json:",omitempty"` // //go:embed patterns
-	TestEmbedFiles     []string `json:",omitempty"` // //files matched by EmbedPatterns
+	TestEmbedFiles     []string `json:",omitempty"` // files matched by TestEmbedPatterns
 	XTestGoFiles       []string `json:",omitempty"` // _test.go files outside package
 	XTestImports       []string `json:",omitempty"` // imports from XTestGoFiles
 	XTestEmbedPatterns []string `json:",omitempty"` // //go:embed patterns
-	XTestEmbedFiles    []string `json:",omitempty"` // //files matched by EmbedPatterns
+	XTestEmbedFiles    []string `json:",omitempty"` // files matched by XTestEmbedPatterns
 }
 
 // AllFiles returns the names of all the files considered for the package.