Merge iostreams from trunk.

- Use `BOOST_ASSERT` instead of `assert`.
- Fix some documentation errors.


[SVN r67665]

libs/iostreams

@@ -1 +1 @@
-Subproject commit a1e27ad28c67ab33b9f3a01fa02c053d24769657
+Subproject commit f4c752cb844a29e5d57dc0ee6b986a93c05d1e42