gh-91317: Document that Path does not collapse initial `//` (GH-32193)

Documentation for `pathlib` says:

> Spurious slashes and single dots are collapsed, but double dots ('..') are not, since this would change the meaning of a path in the face of symbolic links:

However, it omits that initial double slashes also aren't collapsed.

Later, in documentation of `PurePath.drive`, `PurePath.root`, and `PurePath.name` it mentions UNC but:

- this abbreviation says nothing to a person who is unaware about existence of UNC (Wikipedia doesn't help either by [giving a disambiguation page](https://en.wikipedia.org/wiki/UNC))
- it shows up only if a person needs to use a specific property or decides to fully learn what the module provides.

For context, see the BPO entry.