[backport] Change StringContext scaladoc
[backport] Change StringContext scaladoc

See e2a3498 commit message for explanation

Explicit private declaration better than implicit
Thanks to @som-snytt

(cherry picked from commit d071abe8dd9035d8dd246ecb6b207e584b6437fa)

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