Document hard_assert()

[magy00]

Several SDK examples (like hello.c) call hard_assert() but it is apparently not documented (in the PDF version of the C/C++ SDK or in the datasheets).

It's not so clear how "pico/assert.h" actually gets included and which parts of it are public and when to use it instead of assert() from <assert.h> (or <cassert>).

[lurch]

Fixed on develop