builtin: document "any" and "comparable"

Fixes #49927

Change-Id: I8b34cf13b3bc6338309f005648ca3ee6852927f0
Reviewed-on: https://go-review.googlesource.com/c/go/+/368954
Trust: Ian Lance Taylor <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: ianlancetaylor

src/builtin/builtin.go

@@ -91,6 +91,16 @@ type byte = uint8
 // used, by convention, to distinguish character values from integer values.
 type rune = int32
 
+// any is an alias for interface{} and is equivalent to interface{} in all ways.
+type any = interface{}
+
+// comparable is an interface that is implemented by all comparable types
+// (booleans, numbers, strings, pointers, channels, interfaces,
+// arrays of comparable types, structs whose fields are all comparable types).
+// The comparable interface may only be used as a type parameter constraint,
+// not as the type of a variable.
+type comparable comparable
+
 // iota is a predeclared identifier representing the untyped integer ordinal
 // number of the current const specification in a (usually parenthesized)
 // const declaration. It is zero-indexed.