Explanations

Adds details for concatenation (#140) and binary bits (#141).
Huh, interesting; I don't remember seeing a center dot used for that.

Some points:

- I'm not sure if mentioning what some people confuse it with helps for explaining it to everyone.
- Have you considered using the actual symbols (so preferably either a monospace + or a $+$; and a \cdot)?
- Given the general preference for Python as an example language, and the fact that there are specific Python examples in the book that use string concat, it may be worth mentioning that Python is one of those languages where + is concat.