[llvm] [mlir] [NFC][mlir][NVVM] Move op descriptions to NVVMOpsDoc.td (PR #225090)

[Guray Ozen via llvm-commits]

grypp wrote:

> > Reducing the size of the monolithic NVVMOps.td file is not the main motivation for this approach, but it would also be a useful side effect, given that the file is approaching ~8K lines.
> 
> What about splitting it into multiple files, 1 per category: Like arithmetic op, memory op, synchronization, those kinds of things.

+1 

https://github.com/llvm/llvm-project/pull/225090