help.lint.coverage: fix coverage on predicate? words

help.lint.coverage: fix coverage on predicate? words

fix a regression for `predicate?` words revealed by uses of
  `"help" word-prop` over `help:word-help`.

`"help" word-prop` is `f` for `predicate?` words like `array?` and
  `zero-matrix?`. a poorly-written comparison in
  `M\ word <word-help-coverage>` meant that result was effectively
  compared to an empty array { }, instead of just being tested for
  emptiness.

additionally, `(word-help)` is introduced to revive the old behaviour of
  `help:word-help` before the move to split `$inputs` and `$outputs` in
  Help rendering. `(word-help)` provides the actual help content for
  words where `"help" word-prop` is `f`, by calling `word-help*`.