docs(python): Define what a 'character' means in slice / len_chars

[stinodego]

Closes #10349

Credit to @mcrumiller who signalled this issue and started this in #10339. I am splitting this out as the implementation of the str.head/tail should not block this doc improvement.

[mcrumiller]

Ahh geez I forgot about that one. I couldn't figure out how to negative index on head/tail at the time, but now that @reswqa has expressified str.slice we should be able to just piggyback on his work and add those fairly easily.

[stinodego]

Ahh geez I forgot about that one. I couldn't figure out how to negative index on head/tail at the time, but now that @reswqa has expressified str.slice we should be able to just piggyback on his work and add those fairly easily.

I just accepted the head/tail issue - do you want to revisit that PR? The feature is welcome :)

[mcrumiller]

Sure thing @stinodego -- once I finish my "real" work which is usually not as fun.