builtin: document clear is a no-op if its argument's value is nil

Just like the builtin function delete's comment does.

Change-Id: Id94a3aaa03c7b09594bae2b1af901d9060d9e255
GitHub-Last-Rev: 30cbebca96d9099f67a7dfc7650357aa19d6b810
GitHub-Pull-Request: golang/go#70801
Reviewed-on: https://go-review.googlesource.com/c/go/+/635535
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Keith Randall <khr@golang.org>

src/builtin/builtin.go

@@ -247,7 +247,7 @@ func imag(c ComplexType) FloatType
 // 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.
+// implied by the type argument. If t is nil, clear is a no-op.
 func clear[T ~[]Type | ~map[Type]Type1](t T)
 
 // The close built-in function closes a channel, which must be either