gparam: change value of G_PARAM_EXPLICIT_NOTIFY

GParamSpec has a possibility of user-introduced flags, and we didn't
respect that with the addition of _EXPLICIT_NOTIFY.

Change the documentation for the maximum number of user flags to 10,
just to pick a somewhat random number.  The documentation here was never
correct anyway -- it previously claimed that as many as 38 flags were
possible.

Meanwhile, move G_PARAM_EXPLICIT_NOTIFY next to _DEPRECATED in order to
avoid conflicts with low-numbered user flags (which are in use by at
least evolution-data-server).

https://bugzilla.gnome.org/show_bug.cgi?id=731341