Do not print Unknown Field: when there is an unknown field

[tristanlatr]

The expected behaviour, would be, as Sphinx, to just print the field name in bold.
Maybe apply .title().

[mthuurne]

In general I prefer tools to complain loudly when non-existing syntax is used, as that helps discover errors early.

There is the newfield mechanism that can be used to define new fields that pydoctor doesn't know out-of-the-box. It's used in epydoc like this:

:newfield contributor: Contributor, Contributors (Alphabetical Order)
:contributor: `Glyph Lefkowitz  <mailto:[email protected]>`__
:contributor: `Edward Loper  <mailto:[email protected]>`__

[tristanlatr]

Replaced by #456