media: videobuf2-core.h: Document the alloc memop size argument as page aligned

The size argument of the alloc memop, which allocates buffer memory, is
page aligned. Document it as such in the only caller as well as ops
documentation.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>