gen_kobject_list.py: better comments and --help. Zero code change.

- The script can but does not always generate five files, in fact the
  current build invokes it at least three times requesting different
  outputs every time.

- --kobj-types-output produces a code fragment; not a standalone/usable
  header file. It outputs enum constants for a single enum type and not
  several enum types.

- Some outputs include driver instances and others not: clarify which.

- There's an entire and great section in the documentation that took
  me ages to find because it's not referenced anywhere in the --help
  or code. Fixed.

- Highlight the massive duplication in the CMakeLists.txt to save
  déjà vu confusion and minimize future divergence.

- Other minor tweaks.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>