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

src/common/sizer.cpp

@@ -2378,7 +2378,7 @@ void wxBoxSizer::RecalcSizes()
 
             wxCoord maxMajor = GetSizeInMajorDir(item->GetMaxSizeWithBorder());
 
-            // must be nonzero, fixed-size items were dealt with in previous loop
+            // must be nonzero, fixed-size items were dealt with in the previous loop
             const int propItem = item->GetProportion();
 
             // is the desired size of this item small enough?