sync: improve RwLock API docs (#2252)
sync: improve RwLock API docs (#2252)

Currently, the documentation for `tokio::sync::RwLock` states that it
has an unspecified priority policy dependent on the operating system.
This is incorrect: Tokio's `RwLock` is fairly queued. The incorrect
documentation appears to have been copied from the `std::sync::RwLock`
docs, for which this *is* the case.

This commit corrects the documentation to describe the actual priority
policy.

Signed-off-by: Eliza Weisman <f43a79dfd42415a8656f99967c2caaf66ded84db@buoyant.io>
tokio-rs/tokio,tokio-rs/tokio