doc: fix genrest.py to use utf-8 encoding

The genrest.py script (used to create the Kconfig configuration option
documentation) was depending on the environment's locale settings to
properly handle UTF-8 character encoding in the reST files it creates.
When we reused this script for another project, that environment's
LC_CTYPE was set to C, telling Python to use ASCII as the (default)
encoding.  This patch is a prophylactic measure to prevent potential
failures by adding the encoding option on the open() calls.

It also includes a small tweak to the introduction.

Signed-off-by: David B. Kinder <david.b.kinder@intel.com>