The `face-attribute-relative-p' Example?

[Herbert Euler]

In the "Face Attribute Functions" section of Emacs Lisp Reference
Manual (info "(elisp) Attribute Functions"), `read-face-name' is used
as an example for `face-attribute-relative-p':

- Function: face-attribute-relative-p attribute value
    This function returns non-`nil' if VALUE, when used as the value
    of the face attribute ATTRIBUTE, is relative.  This means it would
    modify, rather than completely override, any value that comes from
    a subsequent face in the face list or that is inherited from
    another face.

    `unspecified' is a relative value for all attributes.  For
    `:height', floating point values are also relative.

    For example:

         (read-face-name "Describe face" "= `default' face" t)

    prompts with `Describe face (default = `default' face): '.

What is the relation between the example and the function?
If this example is correct, there should be a rationale at least,
i.m.o.

Regards,
Guanpeng Xu

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