[clang] Document the const and pure attributes (PR #205881)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> Do we want to mention C23 reproducible/unsequenced? (I know we haven't implemented them yet, but it's a useful point of comparison.)

I figured we would mention those if we ever elected to support them. (Right now there are no plans to support those standard attributes due to them being type attributes, but we might support the GCC extension allowing them as declaration attributes, but that's orthogonal to this PR.)

https://github.com/llvm/llvm-project/pull/205881