go/doc: mark comparable predeclared

Add comparable to the list of predeclared types.

Fixes golang/go#51141.

Change-Id: I4a2d4e7e5680e115de9bca03b6c8ad454551cb82
Reviewed-on: https://go-review.googlesource.com/c/go/+/385114
Trust: Jonathan Amsterdam <jba@google.com>
Run-TryBot: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/go/doc/reader.go

@@ -927,6 +927,7 @@ var predeclaredTypes = map[string]bool{
 	"any":        true,
 	"bool":       true,
 	"byte":       true,
+	"comparable": true,
 	"complex64":  true,
 	"complex128": true,
 	"error":      true,

src/go/doc/testdata/b.0.golden

@@ -46,6 +46,9 @@ VARIABLES
 
 
 FUNCTIONS
+	// Associated with comparable type if AllDecls is set. 
+	func ComparableFactory() comparable
+
 	// 
 	func F(x int) int
 

src/go/doc/testdata/b.1.golden

@@ -38,6 +38,12 @@ TYPES
 	// 
 	func (x *T) M()
 
+	// Should only appear if AllDecls is set. 
+	type comparable struct{}	// overrides a predeclared type comparable
+
+	// Associated with comparable type if AllDecls is set. 
+	func ComparableFactory() comparable
+
 	// 
 	type notExported int
 

src/go/doc/testdata/b.2.golden

@@ -46,6 +46,9 @@ VARIABLES
 
 
 FUNCTIONS
+	// Associated with comparable type if AllDecls is set. 
+	func ComparableFactory() comparable
+
 	// 
 	func F(x int) int
 

src/go/doc/testdata/b.go

@@ -27,9 +27,15 @@ func UintFactory() uint {}
 // Associated with uint type if AllDecls is set.
 func uintFactory() uint {}
 
+// Associated with comparable type if AllDecls is set.
+func ComparableFactory() comparable {}
+
 // Should only appear if AllDecls is set.
 type uint struct{} // overrides a predeclared type uint
 
+// Should only appear if AllDecls is set.
+type comparable struct{} // overrides a predeclared type comparable
+
 // ----------------------------------------------------------------------------
 // Exported declarations associated with non-exported types must always be shown.