doc/languages-frameworks/python: Reword section to make commit rules a bit clearer

* `meta.platforms` takes the default value in many cases.

  It does not need to be set explicitly unless the package requires a specific platform.

* The file is formatted with `nixfmt-rfc-style`.

* Commit names of Python libraries should reflect that they are Python

  libraries, so write for example `python311Packages.numpy: 1.11 -> 1.12`.

  It is highly recommended to specify the current default version to enable

  automatic build by ofborg.

* Commit names of Python libraries must reflect that they are Python

  libraries (e.g. `python311Packages.numpy: 1.11 -> 1.12` rather than `numpy: 1.11 -> 1.12`).

* The current default version of python should be included

  in commit messages to enable automatic builds by ofborg.

  For example `python311Packages.numpy: 1.11 -> 1.12` should be used rather

  than `python3Packages.numpy: 1.11 -> 1.12`.

  Note that `pythonPackages` is an alias for `python27Packages`.

* Attribute names in `python-packages.nix` as well as `pname`s should match the

  library's name on PyPI, but be normalized according to [PEP