explain seL4 "bit" sizes more prominently

[Gerwin Klein]

2 queries for the untyped tutorial makes this a frequently asked question (see e.g. #71 ).

We should explain more prominently that those sizes are the exponent n in 2^n bytes. Ideally in the tutorial solution as a comment and in the text.

• add solution comment
• check if untyped is the right tutorial or if this should come earlier and have a reference in untyped
• add text to tutorial itself
• check if this is explained in the reference manual

[Gerwin Klein]

The explanation in the manual is mostly right, but could be a bit clearer, it's phrased in terms of log_2, which is correct but terse.