Books don't have a functional purpose in the same way as software. With a book, there are many ways to state the exact same fact (and the fact itself is not protected by copyright law). With a program, there's only one way to call a particular function, or to write a function declaration that an existing program can call. The exact text used in a program is relevant to its functional purpose.

A more apt analogy in the case of a reference manual is perhaps a competing manual that presented all the same maintenance information with different words, since they would achieve the same functional purpose with different expressive content.