[DebugInfo][docs] Document DILabel in LangRef

Add some minimal documentation for DILabel, originally introduced in
D45024. Update the name and semantics of the `variables:` field in the
documentation for `DISubprogram`; the field is now called
`retainedNodes:` and is a heterogeneous list of `DILocalVariable` and
`DILabel`.

Reviewed By: aprantl

Differential Revision: https://reviews.llvm.org/D89082