[DebugInfo][Docs] Document MIR language debug-info constructs

This patch documents the MIR syntax for a number of things relevant to
debugging information:
 * Trailing 'debug-location' metadata that becomes a DebugLoc,
 * Variable location metadata for stack slots,
 * Syntax for DBG_VALUE metainstructions,
 * Syntax for DBG_INSTR_REF, including trailing instruction numbers
   attached to MIR instructions.

Differential Revision: https://reviews.llvm.org/D89337