MINOR: cli: put @master @<relative pid> @!<pid> in the help

Add help for the prefix command of the CLI. These help only displays
from the CLI of the master.
diff --git a/src/cli.c b/src/cli.c
index f4c5167..4ed8b93 100644
--- a/src/cli.c
+++ b/src/cli.c
@@ -2319,6 +2319,9 @@
 
 /* register cli keywords */
 static struct cli_kw_list cli_kws = {{ },{
+	{ { "@<relative pid>", NULL }, "@<relative pid> : send a command to the <relative pid> process", NULL, cli_io_handler_show_proc, NULL, NULL, ACCESS_MASTER_ONLY},
+	{ { "@!<pid>", NULL }, "@!<pid>        : send a command to the <pid> process", cli_parse_default, NULL, NULL, NULL, ACCESS_MASTER_ONLY},
+	{ { "@master", NULL }, "@master        : send a command to the master process", cli_parse_default, NULL, NULL, NULL, ACCESS_MASTER_ONLY},
 	{ { "help", NULL }, NULL, cli_parse_simple, NULL },
 	{ { "prompt", NULL }, NULL, cli_parse_simple, NULL },
 	{ { "quit", NULL }, NULL, cli_parse_simple, NULL },