Revise documentation in core::ops::bit

Part of #29365.
* Added "real" examples for `BitOrAssign`, `BitXorAssign`, `ShlAssign`,
  and `ShrAssign`
* Rewrote method summary senteces to be in 3rd person singular
* Rephrased example introductions to be less redundant ("in this example"
  etc.) and to not use "trivial"
* Removed superfluous explicit `fn main()`s in examples
* Added some missing periods