Add section on documenting pseudo-patterns

[heitoralthmann]

As discussed in pattern-lab/patternlab-node#921

[bradfrost]

@heitoralthmann you asked:

Do you have a suggestion as to which page this would fit best?

I think this is the right page: https://patternlab.io/docs/pattern-documenting.html and it would be best as its own "Documenting Pseudo-Patterns" heading.

And perhaps in the pseudo-patterns page we can provide a cross link back to this.

Thanks!