[MLIR][SPIRVToLLVM] Updated the documentation for the conversion

This patch updates the SPIR-V to LLVM conversion manual.
Particularly, the following sections are added:
- `spv.EntryPoint`/`spv.ExecutionMode` handling
- Mapping for `spv.AccessChain`
- Change in allowed storage classes for `spv.globalVariable`
- Change of the runner section name

Reviewed By: mravishankar

Differential Revision: https://reviews.llvm.org/D86288