Fix String documentation

[thizanne]

BatString documentation should be taken care of: there is no organisation, some functions are badly documented (right especially refers to left), and so on.

This is a big and ungrateful work, so let's just leave this issue open as a reminder. Maybe other modules could also benefit from some work, but String appeared as quite neglected as I was trying to find a function in the documentation.

[fccm]

The ocamldoc of String doesn't look that bad to me (I don't really see what should be improved).
This comment was written in 2018, so was it improved since then?
Only the functions that was switched to use the new bytes type should maybe have a deprecated tag added so that they could be removed in the future.