topology: Revise document and comments for ABI v5

topology: Revise document and comments for ABI v5

Revise the obsolete content in document and comments.

Here are the major changes in implementation from early phase design:
- PCM object is used to configure front end DAI & DAI links, not used by
  physical DAI or DAI links.
- No longer use separate object for backend or codec<->codec links, but
  use physical links to cover them.

Signed-off-by: Mengdong Lin <mengdong.lin@linux.intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>