Fix and update descriptions

Many parts of Bartimaeus' tweaked descriptions have been incorporated,
along with some additional changes and fixes.

Style changes incorporated include:

- standardizing saving throw blocks
- moving "(x per day)" from abilities row to the individual abilities
for versatility
- prioritizing "THAC0" and "AC" abbreviations over "attack rolls" and
"armor class"
- capitalizing dice and spacing out bonuses (e.g. 1D4 + 1 not 1d4+1)
- removing "points of damage" in favour of "damage"
- removing "additional" qualifier on abilities providing bonus
damage/THAC0 where obvious
- regeneration now always listed as x hp/time
- two linebreaks between paragraphs in flavour text
- Oxford comma for lists
- more commas added to clarify messy sentences
- same pluralization (none) in Usable By section as Not Usable By