ptp: Fix documentation to match code.

Ever since commit 3a06c7ac ("posix-clocks: Remove interval timer
facility and mmap/fasync callbacks") the possibility of PHC based
posix timers has been removed.  In addition it will probably never
make sense to implement this functionality.

This patch removes the misleading text which seems to suggest that
posix timers for PHC devices will ever be a thing.
Signed-off-by: NRichard Cochran <richardcochran@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/ptp/ptp.txt

@@ -18,7 +18,6 @@
     - Adjust clock frequency
 
   + Ancillary clock features
-    - One short or periodic alarms, with signal delivery to user program
     - Time stamp external events
     - Period output signals configurable from user space
     - Synchronization of the Linux system time via the PPS subsystem
@@ -48,9 +47,7 @@
    User space programs may control the clock using standardized
    ioctls. A program may query, enable, configure, and disable the
    ancillary clock features. User space can receive time stamped
-   events via blocking read() and poll(). One shot and periodic
-   signals may be configured via the POSIX timer_settime() system
-   call.
+   events via blocking read() and poll().
 
 ** Writing clock drivers