docs: admin-guide: Add description of %c corename format

There is somehow no description of %c corename format specifier for
/proc/sys/kernel/core_pattern. The %c corename format specifier is
used by user-space application such as systemd-coredump, so it should
be documented.

To find where %c is handled in the kernel source code, look at
function format_corename() in fs/coredump.c.
Signed-off-by: HATAYAMA Daisuke <d.hatayama@fujitsu.com>
Link: https://lore.kernel.org/r/TYAPR01MB4014714BB2ACE425BB6EC6B7951A0@TYAPR01MB4014.jpnprd01.prod.outlook.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/sysctl/kernel.rst

@@ -213,6 +213,7 @@ core_pattern is used to specify a core dumpfile pattern name.
 	%h	hostname
 	%e	executable filename (may be shortened)
 	%E	executable path
+       %c      maximum size of core file by resource limit RLIMIT_CORE
 	%<OTHER> both are dropped
 
 * If the first character of the pattern is a '|', the kernel will treat