Fix spelling and punctuation errors in comments and documentation

Fix several occurrences of misspelling "trivial" as "trival"; use "from"
rather than "than"; add missing articles and commas.

Closes https://github.com/wxWidgets/wxWidgets/pull/874

interface/wx/html/htmlwin.h

@@ -143,7 +143,7 @@ public:
     @event{EVT_HTML_CELL_HOVER(id, func)}
         The mouse passed over a wxHtmlCell.
     @event{EVT_HTML_LINK_CLICKED(id, func)}
-        A wxHtmlCell which contains an hyperlink was clicked.
+        A wxHtmlCell which contains a hyperlink was clicked.
     @endEventTable
 
     @library{wxhtml}
@@ -555,7 +555,7 @@ wxEventType wxEVT_HTML_LINK_CLICKED;
 
     @beginEventTable{wxHtmlLinkEvent}
     @event{EVT_HTML_LINK_CLICKED(id, func)}
-        User clicked on an hyperlink.
+        User clicked on a hyperlink.
     @endEventTable
 
     @library{wxhtml}