Document the `PATH` that `Command` searchs

[ChrisDenton]

Location

std::process::Command

Summary

On all platforms, we search the child's PATH rather than the PATH of the spawning process. This is enforced by this standard library test: https://github.com/rust-lang/rust/blob/ed49386d3aa3a445a9889707fd405df01723eced/library/std/tests/process_spawning.rs

Separately we should also document platform specific behaviour, even if it's subject to change.

This might require libs-api sign off but I feel this should be documented because it's somewhat surprising.