Display $f hypotheses for some wff and class definitions

[GinoGiotto]

Mentioned in: #26 (comment)

I systematically only display the essential hypotheses. I could make an exception for syntax definitions, and display floating hypotheses in this case, in order to match with the metamath pages.

Examples:

• In this example wne has two class hypotheses.
  Official Metamath page: https://us.metamath.org/mpeuni/wne.html
  metamath-web: https://metamath.tirix.org/mpests/wne.html

• In this example wmo has a setvar and wff hypothesis:
  Official Metamath page https://us.metamath.org/mpeuni/wmo.html
  metamath-web: https://metamath.tirix.org/mpests/wmo.html

Beginner question: Can class definitions have $f hypotheses? With a quick search I didn't find any.

EDIT: nvm I found one: https://us.metamath.org/mpeuni/csb.html