document the new page fault output

The global values are still valid, but there are now db level stats as well.

db.serverStatus(); - 2.1.2+

"recordStats" : {
"accessesNotInMemory" : 3,
"pageFaultExceptionsThrown" : 1,
"alerts" :

{ "accessesNotInMemory" : 0, "pageFaultExceptionsThrown" : 0 }

,
"something" :

{ "accessesNotInMemory" : 0, "pageFaultExceptionsThrown" : 0 }

,
"foo" :

{ "accessesNotInMemory" : 0, "pageFaultExceptionsThrown" : 0 }

,
"bar" :

{ "accessesNotInMemory" : 0, "pageFaultExceptionsThrown" : 0 }

,