… variable

- Aims to address [issue sphinx-doc#6478](sphinx-doc#6478)

- Left ObjectDescription._doc_field_type_map declaration alone and in place in
  case there are other factors at play that require it declared on the class.

- In ObjectDescription constructor, explicitly set _doc_field_type_map's value
  as an instance variable.

- This presumably needs to be built with every instantiation of an
  ObjectDescription or inheriting class, so we're not attempting to "define it
  once on a class and then use it"... that approach seems much harder to reason
  about and get correctly done (as was maybe demonstrated by the refactoring
  that lead to this problem in the linked-to issue).

- Runs through the test suite clean locally except for an unrelated single test
  failure on an image_converter test, which also failed on the base branch
  before my changes here.

- type annotation for _process_type_map was incorrect; needed to document the
  positional argument for the method

- __init__ method for ObjectDescription missing type annotation, so added

…ariable

- We can more simply have ObjectDescription.get_field_type_map() make sure that
  it's going to lazily set the value of an instance variable, not the class
  variable, when it populates the field type map.