doc fixes to 'array-protocol type strings'

The item size is not always used (and not always means exact bytes), and
'O' was not in the table.
explain character codes of dtype.kind

"one of 'biufcSUV'" is not very helpful if it stands alone,
also the 'O' typecode was missing.
"(byte-)string" in PyArrayInterface.typekind docs
Merge pull request #4067 from hmeine/master

explain character codes of dtype.kind