Add notes to more accurately describe misaka library extensions

[mgc8]

Added note to the "math" extension to clarify that it doesn't actually render any math, just marks it as ignored by the parser, so that it can be processed with external JavaScript libraries separately. Added notes to the "highlight", "tables" and "underline" extensions with the requirements for each to actually work when used.

Checklist

• [x] All new and existing tests are passing
• [x] (If adding features:) I have added tests to cover my changes
• [x] (If docs changes needed:) I have updated the documentation accordingly.
• [ ] I have added an entry to CHANGES.rst because this is a user-facing change or an important bugfix
• [ ] I have written proper commit message(s)

What changes does this Pull Request introduce?

Documentation changes only -- clarifications on the functionality of misaka extensions.

Why is this necessary?

The existing documentation was incomplete.

[mgc8]

Sorry for the wrong format, I've made the changes locally to squash the commits and re-phrase, then pushed it over the existing PR. I hope it's fine now, please have a look and let me know if other changes are necessary.

[ix5]

Thank you.

One last thing, please fix the commit title length. E.g. docs: markdown-config: Clarify misaka config would be succinct enough.

We'd want to avoid this:

[mgc8]

Thank you for taking the time on the reviews as well.

One last thing, please fix the commit title length. E.g. docs: markdown-config: Clarify misaka config would be succinct enough.

Yeah, the funny thing is that I noticed that, and actually did a "reword" when I squashed the commits and in my local logs it appeared fine; but for some reason it didn't update when I pushed things to GitHub :-/ Could be that my local copy became a bit confused with all the re-basing happening from the other commits that come in, so I deleted everything, re-pulled and re-based again, I hope it's fine now?

[ix5]

Merged, thanks!