[libc] Reorganize and clarify a few points around benchmarking

A few documentation clarifications and moving one part of the
docs around to be closer to the first mention of display so that
it's easier to spot based on some user feedback.

Differential Revision: https://reviews.llvm.org/D79443

GitOrigin-RevId: 880115e65ecd7a8838588faf6dfeef5c37e7f586
1 file changed