doc: remote.*.skip{DefaultUpdate,FetchAll} stops prefetch

doc: remote.*.skip{DefaultUpdate,FetchAll} stops prefetch

Back when 7cc91a2f (Add the configuration option skipFetchAll,
2009-11-09) added for the sole purpose of adding skipFetchAll as a
synonym to skipDefaultUpdate, there was no explanation about the
reason why it was needed., but these two configuration variables
mean exactly the same thing.

Also, when we taught the "prefetch" task to "git maintenance" later,
we did make it pay attention to the setting, but we forgot to
document it.

Document these variables as synonyms that collectively implements
the last-one-wins semantics, and also clarify that the prefetch task
is also controlled by this variable.

Signed-off-by: Junio C Hamano <gitster@pobox.com>