drm/i915/cdclk: Document CDCLK components

Improve documentation by giving an overview of the components involved
in the generation of the CDCLK.

v2: Fix htmldoc error because of missing blank line at the start of
    bulleted list.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Gustavo Sousa <gustavo.sousa@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240221185131.287302-2-gustavo.sousa@intel.com