Emphasize `size_t` Importance for %b Formatting

This pull request solves my complaints in Issue \#120.

If people followed the old example code, it was possible for them to introduce very subtle and very hard to debug bugs with integer types being interpreted as `size_t` within the format parsing code of libhiredis.

I fixed the example code, made the test code follow this standard, and also updated documentation to emphasize the importance of ensuring the length is of type `size_t`.