docs: add docstrings and doctests to generate_all_subsequences

[dwaddle]

Adds the required doctests and docstrings to the backtracking/all_subsequences.py module to ensure it conforms to the repository's testing standards.

[algorithms-keeper]

Closing this pull request as invalid

@dwaddle, this pull request is being closed as none of the checkboxes have been marked. It is important that you go through the checklist and mark the ones relevant to this pull request. Please read the Contributing guidelines.

If you're facing any problem on how to mark a checkbox, please read the following instructions:

• Read a point one at a time and think if it is relevant to the pull request or not.
• If it is, then mark it by putting a x between the square bracket like so: [x]

NOTE: Only [x] is supported so if you have put any other letter or symbol between the brackets, that will be marked as invalid. If that is the case then please open a new pull request with the appropriate changes.