Update docs for $_ and $nil.

pkg/eval/builtin_ns.go

@@ -12,8 +12,7 @@ import (
 //
 // A blackhole variable.
 //
-// Values assigned to it will be discarded. Trying to use its value (like `put $_`)
-// causes an exception.
+// Values assigned to it will be discarded. Referencing it always results in $nil.
 
 //elvdoc:var args
 //
@@ -44,8 +43,6 @@ import (
 //elvdoc:var nil
 //
 // A special value useful for representing the lack of values.
-//
-// **WARNING**: Due to a bug, `$nil` cannot be used as a map key now.
 
 //elvdoc:var paths
 //

pkg/eval/vars/blackhole.go

@@ -12,7 +12,7 @@ func (blackhole) Get() interface{} {
 
 // NewBlackhole returns a blackhole variable. Assignments to a blackhole
 // variable will be discarded, and getting a blackhole variable always returns
-// an empty string.
+// nil.
 func NewBlackhole() Var {
 	return blackhole{}
 }