[llvm][doc] Update comments and documentation of custom stackmap formats in GC

Since https://reviews.llvm.org/D53892 it is possible to emit a custom stackmap by overwriting the emitStackMaps method of GCMetadataPrinter. That way even AOT compilers can generate a more efficient and more suitable format for their needs.

This patch updates documentation and stale comments in source code. In particular it removes the issue from the issue list in the Statepoints documentation and adjusts comments in GCStrategy.

Differential Revision: https://reviews.llvm.org/D119660