Rollup merge of #122916 - MultisampledNight:docs-sync-typo, r=jhpratt

docs(sync): normalize dot in fn summaries

All other functions in e.g. [`Mutex`](https://doc.rust-lang.org/std/sync/struct.Mutex.html) have a dot at the end of their first doc line, except for the newly stabilized [`Mutex::clear_poison`](https://doc.rust-lang.org/std/sync/struct.Mutex.html#method.clear_poison) (and its friend [`RwLock::clear_poison`](https://doc.rust-lang.org/std/sync/struct.RwLock.html#method.clear_poison)).

This PR remedies that by adding a normalizing dot.

Author: matthiaskrgr