LangRef.rst: Clarify how basic blocks without named label are handled.

Describe that they are assigned numbered label using the same counter
as for unnamed temporaries.

Based on http://llvm.org/bugs/show_bug.cgi?id=16043 and mailing list
discussion.

Patch by Paul Sokolovsky!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@182332 91177308-0d34-0410-b5e6-96231b3b80d8
Comment update: these things are called "configuration names" these days, not
"triples". Also remove the implication that they're only used for specifying a
target.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@182335 91177308-0d34-0410-b5e6-96231b3b80d8