Update signature presentation in pinfo classes

classes:

- init definition comes before class docstring
- remove Constructor info block, because it no longer has multiple entries
- don't include instance call definition as definition

objects:

- don't duplicate call def and definition in callables
sentence case

There was a mixture of sentence and title case,
let's be consistent.
simplify title_width expression
Merge pull request #5285 from minrk/constructor-definition

Update signature presentation in pinfo classes