Section 10.16 in the Users Guide. Also reviewed mentions/links from other sections: none need revision.

?github having trouble rendering/previewing the .rst -- that was before I started this edit.

I inserted a sub-title so I could put a link to it: .. _pattern-sig-equiv: "Pattern signature equivalent"
I wanted it to be a low-level title (not a heading), so it doesn't  mess up the section numbering.
But if I underline with tildes or up-arrows (per GHC style guide), github fails to render the whole file.

I've ended up underlining with dashes: not what I want. Help! thank you.

A less damaging way to do the link.

as per comments on the ticket; also linked to Haskell folk art of 'Smart constructors'.