Update explanation of the set_location() method to indicate that in BTree databases, the key need not be in the database.  Also, tell about the exception if the key is not in the DB for other DB types.
Update explanation of the set_location() method to indicate that in
BTree databases, the key need not be in the database.  Also, tell about
the exception if the key is not in the DB for other DB types.

This closes SF bug #127377.

sk-/python2.7-type-annotator,sk-/python2.7-type-annotator,sk-/python2.7-type-annotator