plain test(1) with no arguments is not documented

[Dan Jacobson]

Ah ha, plain
$ test
with no arguments is not documented.  The closest thing documented is
$ test STRING
but when
$ string=; test $string
then sorry. Not documented. man test, help test, info test, /usr/bin/[ --help,
nope, not documented.
If the result is not defined, still document that fact.
And, document plain
$ test -n
which acts differently than plain
$ test

In info one sees,
   Besides the options below, a single argument is also allowed.
But still you haven't mentioned what about no arguments.

BTW, the test manpage perhaps should take a lesson from bash's help
test, and not say recursive things like

EXPRESSION is true or false and sets exit status.  It is one of:

       ( EXPRESSION )
              EXPRESSION is true

until further down the page after first lays out what EXPRESSION is.