std.string doc issue11895

std.algorithm style doc header

https://issues.dlang.org/show_bug.cgi?id=11895

preview:
http://burner.github.io/phobos/phobos-prerelease/std_string.html
The wording is a bit awkward here. What about:
> Not all functions for D string handling are in this module. Functions related to Unicode and ASCII are
> found in std.uni and std.ascii, respectively. Other functions that have a wider generality than just
> strings can be found in std.algorithm and std.range.