Partially consolidate "TOCTOU" explanations

[workingjubilee]

Location

std::fs

Summary

"TOCTOU" is used liberally in the std::fs docs. The explanation of the phrase itself, including the overall concept, should probably be centralized to the top of the module docs. That way individual functions and such could link back, so that each function can instead explain the nature of the specific TOCTOU hazard that it either mitigates or is exposed to.

[xizheyin]

@rustbot claim