Configure: document silent-rules + typo

Explicit the behavior regarding silent-rules (i.e. on by default for
release / off by default for development).
Correct a typo in the comment on -Werror option.

Signed-off-by: Clement Chauplannaz <chauplac@gmail.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>