ref-manual, dev-manual: Edits to runtime package management section.

These changes modify the patch sent by Trevor that essentially
re-wrote this section.  My edits were for consistency only and
style of the book.  No technical information or flow was altered.

I did rename the section to be active to match the other package
related sections.  This caused the link in the ref-manual in the
classes chapter to have to be updated.

(From yocto-docs rev: eb2f950786574b1e90adc673ef00f52a70db9be6)

Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>