custom-face-attributes has no doc string

[Drew Adams]

`custom-face-attributes' has no doc string. However, it is hardly something
internal. It is explicitly mentioned in (elisp) Info node Defining Faces:

 The element's second element, ATTS, is a list of face
 attributes and their values; it specifies what the face
 should look like on that kind of terminal. The possible
 attributes are defined in the value of
 `custom-face-attributes'.

Shouldn't `custom-face-attributes' have a doc string explaining its
structure and purpose?