Simplifications to the upper() and lower() SQL functions.

Simplifications to the upper() and lower() SQL functions.
Updates to documentation on sqlite3_bind_text() and sqlite3_result_text()
to make it clear that users should not try to create strings with
embedded NULs and that if they do the result of expression on those strings
is undefined.  Ticket [57c971fc74524a]

FossilOrigin-Name: 9984cc20ca70b7fb39c0b99580a1317a7b0c9c85