Improve docs for py-version

[cdce8p]

Followup to #5019 and #5024

The current documentation around the py-version setting introduced in 2.11 could be improved.

• Conflicting default values -> Default is autogenerated from sys.version_info. Likely requires an exception to not show a default value Small py-version improvements #5069
• Explain best practices
  • Most users should default to running pylint with the oldest Python version their project supports
  • In some cases it's useful to set py-version to allow running pylint with newer versions. E.g. when pylint is used together with a pre-commit hook and most development happens on a newer Python version.
  • Setting py-version will only hide suggestions for features that aren't included in all (by a project supported) Python versions.