Improve base units table
Improve base units table

- Fixed numerous typographical and puncuation nits.
- Fixed weight → mass.
- Added note about deliberately using grams rather than kilogram
  and aligned existing similar note about celsius with it.
- Call out that `bytes` for bits is not a typo.
- Reworded and -formatted the note for `ratio` as the asterisk seemed
  needless and disrupting the flow.

Signed-off-by: beorn7 <91e32ae1d26bd312b370cf8378a5a83accca90a0@grafana.com>

prometheus/docs,prometheus/docs,prometheus/docs