Rollup merge of #120936 - ripytide:master, r=Amanieu

improve `btree_cursors` functions documentation

As suggested by ``@Amanieu`` (and others) in #107540 (rust-lang/rust#107540 (comment))

Improvements:
- Document exact behavior of `{upper/lower}_bound{,_mut}` with each of the three `Bound` types using unambigous words `{greatest,greater,smallest,smaller,before,after}`.
- Added another doc-example for the `Bound::Unbounded` for each of the methods
- Changed doc-example to use From<[T; N]> rather than lots of `insert()`s which requires a mutable map which clutters the example when `mut` may not be required for the method (such as for `{upper,lower}_bound`.
- Removed `# Panics` section from `insert_{before,after}` methods since they were changed to return an error instead a while ago.
- Reworded some phrases to be more consistent with the more regular `BTreeMap` methods such as calling entries "key-value" rather than "element"s.

Author: matthiaskrgr