doc: Edit the application development common procedures.

Corrected the :envvar: markup. Rewrote the text to be less patronizing.

Change-Id: I09ec541ae41ad682106dc73a6e4495216dfa34d3
Signed-off-by: L.S. Cook <leonax.cook@intel.com>
Signed-off-by: Rodrigo Caballero <rodrigo.caballero.abraham@intel.com>