Use term 'folder code owners' instead of 'non-restricted code owners'

The documentation used two different terms to relate to the code owners
that apply to all files in a folder (code owners that are listed without
'per-file' restriction prefix):

1. folder code owners
2. non-restricted code owners

Make this consistent and use always 'folder code owners' which is the
term that is easier to understand.

Signed-off-by: Edwin Kempin <ekempin@google.com>
Change-Id: Icd23972c1df499769a9c166c96c39281b3a31262