Improve documentation about private name mangling

[picnixz]

Private mangling is quite common if you want to emulate a "private" interface without leaving the possibility of the interface being considered "protected" (in the C++/Java sense). I've seen a few constructions in the standard library that are using this so I think it should be clear (visually speaking) in the docs as well.

I tend to forget about the rule of "no transformation if name only consists of underscores" or "underscores stripped + 1 underscore", so I think it's good to highlight them.

Linked PRs

• gh-120452: improve documentation about private name mangling #120451
• [3.13] gh-120452: improve documentation about private name mangling (GH-120451) #121715
• [3.12] gh-120452: improve documentation about private name mangling (GH-120451) #121716

[Eclips4]

Isn't it covered by the https://docs.python.org/3/reference/expressions.html#expressions (Identifiers (Names) section)?

[picnixz]

Yes it is, but I wanted to improve the wording actually. Currently, it's a one big paragraph and the rules could be separated in a bit more clearly IMO (you can see how it would be rendered on the PR).

[picnixz]

Completed in #120451.