t/op/fork.t: Add description for one special test.
t/op/pow.t: Add descriptions for tests lacking them.

Those which were still lacking descriptions were all testing that
exponentiation has precedence over negation.
t/op/pwent.t: Add descriptions for all tests.
t/op/pos.t: Provide descriptions for remaining tests lacking them.
t/op/or.t: Provide descriptions for tests lacking them.
t/op/oct.t: Add descriptions to tests lacking them.
Add parens around second argument to tests 4 and 5.

If a description were to be added to these tests, in the absence of
parentheses the scalar prototype of CORE::not would enforce a scalar context
onto the balance of the statement, leading to apparently anomalous behavior,
viz., the descriptions would not be printed and test 5 would be reported to
FAIL.
t/op/not.t: Add descriptions to all tests.

Also, add note() before tests 4 and 5 explaining rationale for addition of
parentheses to second arguments.