Mojo: DataPipe: Finish docs/comments in core.h and fix some bugs.

Specifically, document the two-phase APIs and make sure that they
correctly check that the sizes are multiples of the element size.

R=darin@chromium.org

Review URL: https://codereview.chromium.org/136793010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244904 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 244904