pgbench: Improve help output of -I option

Add a description of the step letters to the --help output.

Author: Gurjeet Singh <gurjeet@singh.im>
Reviewed-by: Tristen Raab <tristen.raab@highgo.ca>
Discussion: https://www.postgresql.org/message-id/flat/CABwTF4Xbc=K4tFj5Znc8jx0GCufQa577GCDsWD7=71qDnUEOyQ@mail.gmail.com

src/bin/pgbench/pgbench.c

@@ -873,7 +873,14 @@ usage(void)
 		   "\nInitialization options:\n"
 		   "  -i, --initialize         invokes initialization mode\n"
 		   "  -I, --init-steps=[" ALL_INIT_STEPS "]+ (default \"" DEFAULT_INIT_STEPS "\")\n"
-		   "                           run selected initialization steps\n"
+		   "                           run selected initialization steps, in the specified order\n"
+		   "                           d: drop any existing pgbench tables\n"
+		   "                           t: create the tables used by the standard pgbench scenario\n"
+		   "                           g: generate data, client-side\n"
+		   "                           G: generate data, server-side\n"
+		   "                           v: invoke VACUUM on the standard tables\n"
+		   "                           p: create primary key indexes on the standard tables\n"
+		   "                           f: create foreign keys between the standard tables\n"
 		   "  -F, --fillfactor=NUM     set fill factor\n"
 		   "  -n, --no-vacuum          do not run VACUUM during initialization\n"
 		   "  -q, --quiet              quiet logging (one message each 5 seconds)\n"