pg_upgrade:  trim down --help and doc option descriptions
Previous code had old/new prefixes on option values, e.g.
--old-datadir=OLDDATADIR.  Remove them, for simplicity;  now:
--old-datadir=DATADIR.  Also update docs to do the same.
pg_upgrade:  remove -h option
-h (help) is not needed;  pg_upgrade already supports --help and -?,
which is consistent with other tools.