Currently the documentation of `eldoc-documentation-functions' says this:
* `:thing', VALUE is a short string or symbol designating what is
being reported on. The documentation display engine can elect
to remove this information depending on space contraints;
without saying what VALUE could be. For someone who want to write a
frontend to display eldoc signitures, this is useless. For someone who
want to write a documentation backend for eldoc, this is useless as
well. I think the protocol should at least specify what VALUE could be
expected: signature? full-doc? example? links?
Also contraints is spelled wrong. And on line 705.