Improve documentation of SSL module

[picnixz]

This is an umbrella issue for the points raised in #138269 (comment) that I will restate below:

• Indicate positional-only parameters in SSL docs.
• Indicate which OpenSSL version is required for a given functionality.
• Reword some English if needed (I don't know if it's needed but maybe).
• Fix Sphinx references warnings.

cc @ronf

Linked PRs

• gh-138633: synchronize documented signatures of SSL objects with runtime ones #138639
• [3.14] gh-138633: synchronize documented signatures of SSL objects with runtime ones (GH-138639, GH-138648) #138646 (held for now, can wait)
• [3.13] gh-138633: synchronize documented signatures of SSL objects with runtime ones (GH-138639, GH-138648) #138647 (held for now, can wait)
• gh-138633: fix Sphinx references in ssl documentation #138648
• gh-138633: indicate SSL backend requirements for SSL functions #138656