Add a section on Strings and Unicode

I'm not proud of wording but we need clarifications on how exactly builtins correlate to Unicode standard (if at all).
The "obvious meaning" present there previously is not good enough definition.

Along the way I fixed a bug in docs - H5 macro was used but not defined, effectively throwing away level-5 titles.

Why wouldn't DDoc complain about undefined macros instead of sliently eating them, that's a topic in its own right.

Preview of that page:
http://blackwhale.github.io/arrays.html#strings_unicode