Add third type to doc-formal

[DaddyWesker]

Pre-pr behavior:

If some grounded function (or function, written in strdlib.metta) have implementation, but no documentation, then calling help! on it produces error:

> !(help! not)
[(Error (Error (@doc-formal (@item not) (@kind function) (@type (-> Bool Bool)) (@desc "No documentation")) IncorrectNumberOfArguments) "Cannot match @doc-formal structure"), (Error (Error (@kind function) BadType) "Cannot match @doc-formal structure")]

Which comes from get-doc-function, which implementation is:

(: get-doc-function (-> Atom Type %Undefined%))
(= (get-doc-function $name $type)
  (let $top-space (mod-space! top)
  (unify $top-space (@doc $name $desc (@params $params) $ret)
    (let $type' (if (== $type %Undefined%) (undefined-doc-function-type $params) (cdr-atom $type))
    (let ($params' $ret') (get-doc-params $params $ret $type')
      (@doc-formal (@item $name) (@kind function) (@type $type) $desc (@params $params') $ret')))
    (@doc-formal (@item $name) (@kind function) (@type $type) (@desc "No documentation")) )))

As it can be seen, on "else" branch of unify (@doc-formal (@item $name) (@kind function) (@type $type) (@desc "No documentation")) is being returned, but doc-formal breaks since there is only two types:

(: @doc-formal (-> DocItem DocKindFunction DocType DocDescription DocParameters DocReturn DocFormal))
(: @doc-formal (-> DocItem DocKindAtom DocType DocDescription DocFormal))

and none of them could be applied to this case. So, third type is being added here to prevent error.

Post-pr behavior:

> !(help! not)
Function not (type (-> Bool Bool)) No documentation
[()]

(note that I've commented out not's documentation in sake of demonstration).