media: rc-core.rst: add an introduction for RC core

The RC core does several assumptions, but those aren't documented
anywhere, with could make harder for ones that want to understand
what's there.

So, add an introduction explaining the basic concepts of RC and
how they're related to the RC core implementation.

Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>