crimson/common: add comment to explain the partial specialization

crimson/common: add comment to explain the partial specialization

it might be confusing why we don't use explicit specialization for
defining errorator::futurize::stored_to_future.

quote from item 16, § 17.7.3, n4659:

In an explicit specialization declaration for a member of a class
template or a member template that appears in namespace scope, the
member template and some of its enclosing class templates may remain
unspecialized, except that the declaration shall not explicitly
specialize a class member template if its enclosing class templates are
not explicitly specialized as well.

Signed-off-by: Kefu Chai <kchai@redhat.com>