Documentation/sysctl: coredump: add %F for pidfd number

In commit b5325b2a270f ("coredump: hand a pidfd to the usermode coredump
helper") a new core_pattern specifier, %F, was added to provide a pidfs
to the usermode helper process referring to the crashed process.

Update the documentation to include the new core_pattern specifier.

Signed-off-by: Salvatore Bonaccorso <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/admin-guide/sysctl/kernel.rst

···
 	%E		executable path
 	%c		maximum size of core file by resource limit RLIMIT_CORE
 	%C		CPU the task ran on
+	%F		pidfd number
 	%<OTHER>	both are dropped
 	========	==========================================