[llvm] [Docs][AMDGPU] Consistent use of "uniform", "per-lane" and "imm" (PR #209974)

[Sameer Sahasrabuddhe via llvm-commits]

ssahasra wrote:

> I'd vote to write out "immediate" instead of "imm" so that, in case someone who is less familiar with that concept stumbles over it, they have a better term to search for.

Fixed.

https://github.com/llvm/llvm-project/pull/209974