Add documentary names to callback function signatures

[michaelhkay]

If PR #1696 is accepted we can add documentary names to the parameters of callback function signatures, for example fn:filter can become

fn:filter(
  $input as item()*, |  
  $predicate as fn($item as item(), $position as xs:integer) as xs:boolean? |  
) as item()*

and we can (if we need to) use the parameter names in the prose

[ChristianGruen]

Looks good. Related: #981.