* fixed link typos and copy-paster errors
* rewrote Fn* explanations
* `RHS = Self` -> `RHS` is `Self` (added that to all applicable places as
well)
* fixed up some links
* s/MutDeref/DerefMut
* removed remaining superfluous `fn main()`s
* fixed some minor phrasings and factual errors and inaccuracies
std::ops docs: Fix phrasing and factual errors/inaccuracies
Part of #29365.
* Strenghtened summary/explanation split, making phrasings more parallel
* Added links throughout
* Fixed some example formatting & removed extraneous `fn main()`s (or hid
then when needed because of `#![features]`.
* Emphasized note on `RangeFrom`'s `Iterator` implementation
* Added summary sentences to (unstable) `contains` methods
Replaced by adding extra imports, adding hidden code (`# ...`), modifying
examples to be runnable (sorry Homura), specifying non-Rust code, and
converting to should_panic, no_run, or compile_fail.
Remaining "```ignore"s received an explanation why they are being ignored.
