Merge renderChildren() and renderHTMLChildren()

Summary: `renderChildren()` now returns array which isn't ideal but I prefer it to having two methods.

Test Plan: None.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4910

src/docs/developer/rendering_html.diviner

@@ -124,14 +124,6 @@ return `phutil_tag()` or `javelin_tag()`:
 
   return phutil_tag('div', ...);
 
-@{class:AphrontView} subclasses can use `renderHTMLChildren()` and
-`renderSingleView()` to build @{class@libphutil:PhutilSafeHTML} objects from
-children or arbitrary lists of components.
-
-@{class:AphrontView} subclasses should avoid `renderChildren()` and transition
-callers to `renderHTMLChildren()`. This older method does not return
-@{class@libphutil:PhutilSafeHTML} objects.
-
 = Internationalization: pht() =
 
 The @{function:pht} function has some special rules. If any input to