improving documentation

slight typo fix when discussing the need for a bounds argument
pad line_height computations to account for characters with descenders

Calculating vertical positioning based on line_height alone wasn’t
quite working because some characters have descenders – those little
tail-like things on lowercase characters like j, y, g, p, and q. If you
vertically position things strictly according to the line-height,
you’re calculating against the baseline of the text line, which is
above the descender. To account for this, we’re multiplying the final
line_height calculation by 1.5, so the last line will not be rendered
unless there is also room for any descenders that might exist. In the
future we might actually look for descenders before deciding to
increase the required space.