The current docstrings were not easy to understand.

- The previous solution, which used tuples to collect those data, was
easy to break, because it required to put data in the right order; and
undocumented, because it was unclear what kind of data had to be added
in the tuple elements.
- Those limitations made difficult to reason about shortcuts-related
code.

- Before we only allowed class methods decorated with on_conf_change to
do that.
- But that's too limited if we need to use regular functions to observe
an option. And that's precisely what this new method allows us to do.

This is necessary to add observers for specific shortcuts.

- That allows those shortcuts to be updated on the fly when they are
changed in Preferences or directly with set_conf.
- Also, fix inheritance of classes that inherit from
SpyderShortcutsMixin to accomodate this change.

That was introducing an error in test_shortcut_for_widget_is_updated
when run on CI due to the incorrectly named section.

This is to have feature parity of kwargs with the other methods of
SpyderShortcutsMixin.

…public

That's because it's going to be used in other places, so it's not good
to leave it as private in our API.