[MIRLangRef] Document MachineOperand comments

[SjoerdMeijer]

Summary

Late follow-up of D74306 to document MachineOperand comments in MIRLangRef.

[dmgreen]

Sounds good to me.

(Although, I would move it out from the middle of the "Operands" section. Maybe to after Predicate Operands, so it's still in "Machine Operands" but not between two other Operand types)

[SjoerdMeijer]

Thanks Dave, I will move it before committing.

llvm/docs/MIRLangRef.rst

 .. TODO: Describe the syntax of the stack object machine operands and their
    YAML definitions.
 .. TODO: Describe the syntax of the block address machine operands.
 .. TODO: Describe the syntax of the metadata machine operands, and the
    instructions debug location attribute.
 .. TODO: Describe the syntax of the register live out machine operands.
 .. TODO: Describe the syntax of the machine memory operands.
 
+Comments
+^^^^^^^^
+
+Machine operands can have C/C++ style comments, which are annotations enclosed
+between ``/*`` and ``*/`` to improve readability of e.g. immediate operands.
+In the example below, ARM instructions EOR and BCC and immediate operands
+``14`` and ``0`` have been annotated with their condition codes (CC)
+definitions, i.e. the ``always`` and ``eq`` condition codes:
+
+.. code-block:: text
+
+  dead renamable $r2, $cpsr = tEOR killed renamable $r2, renamable $r1, 14 /* CC::always */, $noreg
+  t2Bcc %bb.4, 0 /* CC:eq */, killed $cpsr
+
+As these annotations are comments, they are ignored by the MI parser.
+Comments can be added or customized by overriding InstrInfo's hook
+``createMIROperandComment()``.
+
 Debug-Info constructs
 ---------------------
 
 Most of the debugging information in a MIR file is to be found in the metadata
 of the embedded module. Within a machine function, that metadata is referred to
 by various constructs to describe source locations and variable locations.
 
 Source locations