(Major Mode Conventions): Mention "system abbrevs".

(Major Mode Conventions): Mention "system abbrevs".
Mode inheritance applies only when default-major-mode is nil.
Clarifications.
(Example Major Modes): Update Text mode and Lisp mode examples.
(Minor Mode Conventions): Mention define-minor-mode at top.
(Defining Minor Modes): In Hungry example, don't define C-M-DEL.
(Mode Line Format): Update mode line face display info.
(Properties in Mode): Mention effect of risky vars.
(Imenu): Define imenu-add-to-menubar.
(Font Lock Mode): Add descriptions to menu lines.
(Faces for Font Lock): Add font-lock-doc-face.