Fix inaccurate documentation of wait_while, wait_while_until & wait_while_for

[Jupeyy]

The current documentation can be interpreted incorrectly. For example if you wait for a condition that is already false, the current documentation suggests that the current thread will still block.

see rust-lang/rust#121405

[Amanieu]

I think the new docs are actually harder to read than the old ones. Maybe it might be worth copying the latest docs from the standard library's Condvar if they are clearer: https://doc.rust-lang.org/nightly/std/sync/struct.Condvar.html#method.wait