Signature: Fix documentation

`fixSignature` no longer exists. Instead, we expect the users of
signatures to not cache underdefined signatures.

Author: smarter

compiler/src/dotty/tools/dotc/core/Signature.scala

@@ -87,14 +87,15 @@ case class Signature(paramsSig: List[TypeName], resSig: TypeName) {
 
   /** Construct a signature by prepending the signature names of the given `params`
    *  to the parameter part of this signature.
+   *
+   *  Like Signature#apply, the result is only cacheable if `isUnderDefined == false`.
    */
   def prepend(params: List[Type], isJava: Boolean)(implicit ctx: Context) =
     Signature(params.map(p => sigName(p, isJava)) ++ paramsSig, resSig)
 
   /** A signature is under-defined if its paramsSig part contains at least one
    *  `tpnme.Uninstantiated`. Under-defined signatures arise when taking a signature
-   *  of a type that still contains uninstantiated type variables. They are eliminated
-   *  by `fixSignature` in `PostTyper`.
+   *  of a type that still contains uninstantiated type variables.
    */
   def isUnderDefined(implicit ctx: Context) =
     paramsSig.contains(tpnme.Uninstantiated) || resSig == tpnme.Uninstantiated
@@ -116,7 +117,12 @@ object Signature {
    */
   val OverloadedSignature = Signature(List(tpnme.OVERLOADED), EmptyTypeName)
 
-  /** The signature of a method with no parameters and result type `resultType`. */
+  /** The signature of a method with no parameters and result type `resultType`.
+   *
+   *  The resulting value is only cacheable if `isUnderDefined == false`,
+   *  otherwise the signature will change once the contained type variables have
+   *  been instantiated.
+   */
   def apply(resultType: Type, isJava: Boolean)(implicit ctx: Context): Signature = {
     assert(!resultType.isInstanceOf[ExprType])
     apply(Nil, sigName(resultType, isJava))