login.conf(5): Default values: Rename column, elaborate on absence of such

Column "Notes" in fact only contains default values for capabilities, so
make this clear by renaming it to "Default".

Add a small introductory text mentioning it, and what an absence of
default value means (inheritance).

PR: 271748
Reviewed by: emaste
Approved by: emaste (mentor)
MFC after: 3 days
Sponsored by: Kumacom SAS
Differential Revision: https://reviews.freebsd.org/D40347