[GC Docs] Update LangRef to link to Statepoint docs

Add a brief section linking to the experimental statepoint intrinsics analogous to the one we have linking to patchpoint.  

While I'm here, cleanup some wording about what the gc "name" attribute actually means.  It's not the name of a *collector* it's the name of the *strategy* which may be compatible with multiple collectors.

llvm-svn: 230576