Improve internal documentation of skills system

[endolith]

Describe the changes you have made:

• Add docstrings for skills functions.
• If skills are disabled, don't list skills, return an empty list and a message explaining the situation.

Reference any relevant issues (e.g. "Fixes #000"):

When OI tries to list skills, it shows them, but then it can't actually call them and it gets very confused.

OI tries to call help(computer.skills.import_skills) or help(computer.skills.list), for instance and there is no documentation.

Pre-Submission Checklist (optional but appreciated):

• [ ] I have included relevant documentation updates (stored in /docs)
• [x] I have read docs/CONTRIBUTING.md
• [ ] I have read docs/ROADMAP.md

OS Tests (optional but appreciated):

• [x] Tested on Windows
• [ ] Tested on MacOS
• [ ] Tested on Linux