Sort void elements alphabetically, document ones that don't match cur…

…rent standard (#563)

html5lib/constants.py

@@ -557,24 +557,36 @@
 )
 
 voidElements = frozenset([
+    "area",
     "base",
-    "command",
-    "event-source",
-    "link",
-    "meta",
-    "hr",
     "br",
-    "img",
-    "embed",
-    "param",
-    "area",
     "col",
+    "command",  # removed ^1
+    "embed",
+    "event-source",  # renamed and later removed ^2
+    "hr",
+    "img",
     "input",
+    "link",
+    "meta",
+    "param",  # deprecated ^3
     "source",
     "track",
     "wbr",
 ])
 
+# Removals and deprecations in the HTML 5 spec:
+# ^1: command
+#     http://lists.whatwg.org/pipermail/whatwg-whatwg.org/2012-December/038472.html
+#     https://github.com/whatwg/html/commit/9e2e25f4ae90969a7c64e0763c98548a35b50af8
+# ^2: event-source
+#     renamed to eventsource in 7/2008:
+#     https://github.com/whatwg/html/commit/d157945d0285b4463a04b57318da0c4b300a99e7
+#     removed entirely in 2/2009:
+#     https://github.com/whatwg/html/commit/43cbdbfbb7eb74b0d65e0f4caab2020c0b2a16ff
+# ^3: param
+#     https://developer.mozilla.org/en-US/docs/Web/HTML/Element/param
+
 cdataElements = frozenset(['title', 'textarea'])
 
 rcdataElements = frozenset([