doc: expand list of directories in Linux guide

doc: expand list of directories in Linux guide

Not all directories were given in the GSG document, but many of those
omitted would be of interest to users, e.g. "doc", "license" and
"usertools" directories. Adding these leaves only "devtools", and
"kernel" as the only undocumented directories, so add them in too for
completeness.

When updating the section, add "including" to the line leading up to the
directory list, indicating that, while the list is currently complete,
it is not guaranteed to always be.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>