no bug - fx doc: add a few more std::vector to nsTArray mappings r=mhowell DONTBUILD

Differential Revision: https://phabricator.services.mozilla.com/D186349

xpcom/docs/collections.rst

@@ -20,6 +20,9 @@ std::vector<T>     nsTArray<T>
 ``insert()``       ``AppendElements()``
 ``emplace_back()`` ``EmplaceBack()``
 ``clear()``        ``Clear()`` or ``ClearAndRetainStorage()``
+``data()``         ``Elements()``
+``at()``           ``ElementAt()``
+``back()``         ``LastElement()``
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 Rust Bindings