[MLIR][SPIRVToLLVM] Updated documentation for SPIR-V to LLVM conversion

Updated the documentation for SPIR-V to LLVM conversion, particularly:
- Added a section on control flow
- Added a section on memory ops
- Added a section on GLSL ops

Also, moved `spv.FunctionCall` to control flow section. Added a new section
that will be used to describe the modelling of runtime-related ops.

Reviewed By: antiagainst

Differential Revision: https://reviews.llvm.org/D84734