org-agenda.el: Rename the "top-category" filter as "top-headline"

org-agenda.el: Rename the "top-category" filter as "top-headline"

* org-agenda.el (org-agenda-top-headline-filter): Rename from
`org-agenda-top-headline-filter'.
(org-find-top-headline): Rename from `org-find-top-category'.
Add a docstring.
(org-agenda-filtered-by-top-headline): Rename from
`org-agenda-filtered-by-top-category'.
(org-agenda-filter-by-top-headline): Rename from
`org-agenda-filter-by-top-category'.  Fix docstring.
(org-agenda-filter-top-headline-apply): Rename from
`org-agenda-filter-top-category-apply'.  Fix docstring.
(org-agenda-mode-map): Update binding.

Using category here was confusing, because there is no Org
category involved, the filtering is really against a headline.
Since category filtering also exists, the confusion was bad.