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

[miss-islington]

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)
• 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.
(cherry picked from commit 78f1a43)

Co-authored-by: Oleg Iarygin [email protected]

Automerge-Triggered-By: GH:brettcannon

[miss-islington]

Status check is done, and it's a success ✅ .

[miss-islington]

Status check is done, and it's a success ✅ .

[miss-islington]

Status check is done, and it's a success ✅ .

[miss-islington]

Status check is done, and it's a success ✅ .

[miss-islington]

Status check is done, and it's a success ✅ .