More missing `versionadded` annotations

[dgw]

While looking up some things in the 8.0 docs for sopel-irc/sopel-help#10, I found that AbstractBot.safe_text_length() is missing a versionadded annotation. There are probably other pieces of the API that are missing these important version notes, too. We already did at least one reasonably big documentation pass to clean up stuff like this, but obviously there were omissions (or just new things that were added without annotations after the cleanup had been done).

I expect this will take someone a bit of time, so I don't want to shove it into 8.0.0 and take our collective attention away from finalizing the changelog, upgrade guide, and one last tweak to adminchannel (#2601). Still, I don't want us to forget about it.

Documentation improvements like this are perfect candidates for patch releases, so 8.0.1 it is.

[dgw]

#2642 merged, taking care of the main impetus for this. Further efforts to be left for 8.1; re-milestoned accordingly.