- remove the exclusion of cx_oracle.STRING from setinputsizes by

- remove the exclusion of cx_oracle.STRING from setinputsizes by
configuring cx_oracle.UNICODE on OracleNVarChar.  Attempts
were made to pass unicode data to/from a plain VARCHAR2 with
cx_oracle, both with and without setinputsizes in use, but
it doesn't appear to be possible - therefore users will need to use
Unicode/UnicodeText with oracle if data contains non-ASCII info.
[ticket:1517]
- updated the Unicode/UnicodeText docs to reflect this, that
convert_unicode might not be enough.
- allowed convert_unicode='force' to be significant for bind parameters
as well.