Document setmaxregister launch-bound requirement

[XFDG]

Summary

• document the register-count and warpgroup requirements of the public setmaxregister APIs
• warn that ptxas can ignore setmaxnreg with C7508 when the entry register count is unknown
• show how min_blocks_per_mp establishes a launch bound, while calling out its occupancy and spilling tradeoffs

Testing

• B200 / SM100a, CUDA 13.1 ptxas: without minnctapersm, C7508 is emitted and SASS contains 0 USETMAXREG instructions
• B200 / SM100a, CUDA 13.1 ptxas: with minnctapersm 1, C7508 disappears and SASS contains 2 USETMAXREG instructions
• ruff check --ignore E731 on both changed files (the ignored E731 findings are pre-existing in nvvm_wrappers.py)
• git diff --check

Contribution disclosure

This contribution was prepared with AI-assisted tooling. I reviewed the changes and validated the documented behavior on B200.

Fixes #3406