Correct docs for "kdb5_util load" switches

Remove obsolete parenthetical statement in the description of the
"-update" switch to "kdb5_util load".  Using the "-update" flag to
load a second dump made using the "-ov" switch is no longer necessary
now that the "-r13" format is the default.  Add statement that the
"-b7" dump format was the default for versions prior to 1.2.2 (to
match the statement from "dump" section).

[tlyu@mit.edu: commit message]

ticket: 7865
tags: pullup
target_version: 1.12.2