Incorrect terminology in Customize doc

[Drew Adams]

Some of the Customize documentation incorrectly refers to user
preferences, which can be either faces or variables, as "user
options". In Emacs terminology (elsewhere, and longstanding), a "user
option" is a `user-variable-p', that is, a variable that users are
intended to change (its doc string has "*" at the beginning).

The correct term for a user preference that includes both variables
and faces is "preference", not "option" or "user option". The
Customize doc is confusing enough without this additional confusion of
terms. The doc and the source code comments should be corrected
accordingly.