Clarify scala.collection.immutable.Map#withDefaultValue() docs.
Clarify scala.collection.immutable.Map#withDefaultValue() docs.

Copy the documentation from Map#withDefault() that get(), contains(),
iterator(), and keys() keys are not affected by withDefaultValue().

shimib/scala,scala/scala,scala/scala,jvican/scala,jvican/scala,martijnhoekstra/scala,lrytz/scala,scala/scala,slothspot/scala,shimib/scala,martijnhoekstra/scala,scala/scala,martijnhoekstra/scala,felixmulder/scala,scala/scala,lrytz/scala,scala/scala,martijnhoekstra/scala,lrytz/scala,martijnhoekstra/scala,slothspot/scala,jvican/scala,shimib/scala,shimib/scala,shimib/scala,felixmulder/scala,lrytz/scala,felixmulder/scala,jvican/scala,slothspot/scala,martijnhoekstra/scala,slothspot/scala,slothspot/scala,felixmulder/scala,felixmulder/scala,felixmulder/scala,slothspot/scala,jvican/scala,lrytz/scala,felixmulder/scala,jvican/scala,jvican/scala,lrytz/scala,slothspot/scala,shimib/scala