docs/source/Configuration.rst: Put extra clarification about /etc/avocado/conf.d

Let's reinforce to users that there is a conf.d directory
and it is not used only for extra plugin configuration,
in fact, it can hold any number of new config files, and
all of them will be parsed if valid.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>