Add warning on test -a, -o documentation

[積丹尼 Dan Jacobson]

On the bash man page at
              expr1 -a expr2
                     True if both expr1 and expr2 are true.
              expr1 -o expr2
                     True if either expr1 or expr2 is true.
and on "help test" perhaps add the same warning as one sees on
(info "(coreutils) Connectives for test").