Add docstring to `String` (#43943)
The type `String` itself did not have a docstring, instead the type is
documented through two of its constructors.
Add docstring to `String`, and remove redundant information from the
constructor docstrings.
In particular, mention that `String`s, while they are interpreted as being
UTF8 encoded, do not assume strings are valid UTF8, and may be arbitrary
byte sequences.
