builtin: add documentation for clear

Updates #56351

Change-Id: I7b1dd29e60f71d804a6d9cba3377e28c3a5b964e
Reviewed-on: https://go-review.googlesource.com/c/go/+/462935
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/builtin/builtin.go

@@ -227,6 +227,15 @@ func real(c ComplexType) FloatType
 // the type of c.
 func imag(c ComplexType) FloatType
 
+// The clear built-in function clears maps and slices.
+// For maps, clear deletes all entries, resulting in an empty map.
+// For slices, clear sets all elements up to the length of the slice
+// to the zero value of the respective element type. If the argument
+// type is a type parameter, the type parameter's type set must
+// contain only map or slice types, and clear performs the operation
+// implied by the type argument.
+func clear[T ~[]Type | ~map[Type]Type1](t T)
+
 // The close built-in function closes a channel, which must be either
 // bidirectional or send-only. It should be executed only by the sender,
 // never the receiver, and has the effect of shutting down the channel after