- The method part of the `length` assertion was slated for deprecation
  due to a compatibility issue in legacy environments. The decision to
  deprecate `length` was reversed per chaijs#684. This commit replaces the
  deprecation notice with a recommendation to favor `lengthOf` over
  `length` due to the compatibility issue.

- The `lengthOf` assertion wasn't a true alias of `length` because it
  was a method assertion instead of a chainable method assertion. This
  commit changes `lengthOf` into a chainable method assertion that's
  identical to `length`, and updates tests and docs accordingly.

- Updates docs to classify `length` as an alias of `lengthOf`.

- Updates docs of related assertions to use `lengthOf` instead of
  `length`.

When the `length` assertion is chained directly off of an uninvoked
method, it references `function`'s built-in `length` property instead
of Chai's `length` assertion. This commit adds a guard to Chai methods
to detect this problem and throw a helpful error message that advises
the user on how to correct it.