sizeof confusion: drive home that it's done at compile time

[chmullig]

So many people are super confused by sizeof, and use it where they shouldn't. for example, in place of strlen. It might help some to emphasize that sizeof is (normally ¹) an operator evaluated at compile time, and becomes a constant. So it doesn't know. See K&R pgs 135, and to a lesser extent 203, for more.

1. Note this breaks, slightly, with C99 variable length arrays where sizeof acts like an expression, not a constant. (invalid in the C++ spec as far as I can tell)

[hmontero1205]

runtime vs. compile time kind of out of scope
instead, show diff between strlen and sizeof in same example