Revive and enhance OneToOneField documentation

[Ramiro Morales]

A follow-up to [8757]:

• topics/db/models.txt:
  • Replace OneToOneField section content with a general description copied from pre-docs-rf model-api.txt, with links to the modeltests model example and the field options reference.
  • Fixes some section heading levels.
• topics/db/queries.txt:
  • Replace the last remaining occurrence of the "The semantics of one-to-one relationships will be changing soon, so we don't recommend you use them." warnings with content moved from topics/db/models.txt, it seems a better place for it because it is an example using the Blog-Author-Entry theme used in this document.
• ref/models/fields.txt (OneToOneField):
  • Fixes formatting in the field reference so the parent_link option description is rendered.