usermanual.xml: Two new sections added to BitBake "Description"

1. Added a new section "Appending and Prepending (override style
   syntax)".  This section shows how the append and prepend operators
   work using the override style syntax.

2. Added a new section "Removing (override style syntax)".  This
   section describes the new "_remove" operator.

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