Rollup merge of #104298 - tbu-:pr_set_extension_caveats, r=m-ou-se

Add notes and examples about non-intuitive `PathBuf::set_extension` behavior

Basically, passing the empty string will actually remove the extension instead of setting it to the empty string. This might change what is considered to be an extension. Additionally, passing an extension that contains dots will make the path only consider the last part of it to be the new extension.

diff --cc library/std/src/path.rs
Simple merge