init: cleanup exec_service documentation

The exec_service documentation was difficult to read, clarify it.

Tests:
Run grip.py to verify that the markdown still works correctly.
Run aspell to verify spelling.

Change-Id: I29bdd456f3d3ea2a91c9d4772bd09a5a195f97a9
Signed-off-by: William Roberts <william.c.roberts@intel.com>