doc: add reference to hasAttr in ? operator (#9185)

Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>

doc/manual/src/language/operators.md

@@ -59,6 +59,8 @@ An attribute path is a dot-separated list of [attribute names](./values.md#attri
 Test whether [attribute set] *attrset* contains the attribute denoted by *attrpath*.
 The result is a [Boolean] value.
 
+See also: [`builtins.hasAttr`](@docroot@/language/builtins.md#builtins-hasAttr)
+
 [Boolean]: ./values.md#type-boolean
 
 [Has attribute]: #has-attribute