bug#7136: Documentation: `completion-in-region-functions' punts to opacity

[MON KEY]

Docstring of variable `completion-in-region-functions' has this:

,----
| NEXT-FUN is a function of four arguments (START END COLLECTION PREDICATE)
| that performs the default operation.  The other four arguments are like
| the ones passed to `completion-in-region'.
`----

The punting in the second sentence above (e.g. "The other four") is
not helpful in the least.  The fourth arg PREDICATE is not documented
in `completion-in-region'.

Examining the source for `completion-in-region' one finds its first
evaluated form is a call to the `with-wrapper-hook' macro with
`completion-in-region-functions' as its first arg.

That `with-wrapper-hook' is itself recursive `labels' call with lexical
closures over its args renders the `completion-in-region-functions'
docstring completely opaque.

--
/s_P\