Call things inside square brackets 'elements'.

It doesn't quite fit for section markup, but 'markup' was a terrible name (it's
all markup).


[SVN r67735]

tools/quickbook

@@ -1 +1 @@
-Subproject commit 6daf40dd41d47cd1fc8f58a748a1f0fd719e4054
+Subproject commit 7a1895c4b214ba0b48a9d2d6c8f2dff22d6e6456