The standard says that the initial number of buckets in an unordered
associative container is implementation-defined when not set by the
user (I would have made it unspecified but noone asked me ;-) so this
patch documents our implementation's choice.

        * include/bits/unordered_map.h: Fix typo in comments.
        * doc/xml/manual/status_cxx2011.xml: Add implementation-defined bucket
        counts for unordered associative containers.

Committed to trunk.