Document db.killOp() with Foreground Index Operations

The db.killOp() method can now be used to kill foreground index operations. Add this to the release notes.

Also, remove from the 2.4 manual the mentions that db.killOp() can't kill foreground index ops. The mentions are found in:
source/reference/current-op.txt
source/reference/limits.txt
source/reference/method/db.killOp.txt