Improvements to html:oop output

[simonjwright]

I took some time to understand the meanings of the subprogram headings:

• Declared dispatching subprograms: these are not overriding subprograms. Not sure whether others would find "'not overriding' subprograms" a more useful title.
• Overrided dispatching subprograms:
  • typo, should be "Overridden". I wouldn’t feel bound to alter all the other occurrences!
  • as above, "'overriding' subprograms"
• Inherited prefix-callable subprograms: not sure what benefits this brings, since in my attempt all the entries appear in "Declared prefix-callable subprograms".