Change recovery_init_sync_method to PGC_SIGHUP.

The setting has no effect except during startup.  It's still nice to be
able to change it dynamically, which is expected to be pretty useful to
an admin following crash recovery when restarting the cluster is not so
appealing.

Per discussions following commits 2941138e6 and 61752afb2.

Author: Justin Pryzby <pryzby@telsasoft.com>
Reviewed-by: Fujii Masao <masao.fujii@oss.nttdata.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Thomas Munro <thomas.munro@gmail.com>
Discussion: https://postgr.es/m/20210529192321.GM2082%40telsasoft.com

doc/src/sgml/config.sgml

@@ -9929,7 +9929,8 @@ dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
         appear only in kernel logs.
        </para>
        <para>
-        This parameter can only be set at server start.
+        This parameter can only be set in the
+        <filename>postgresql.conf</filename> file or on the server command line.
        </para>
       </listitem>
      </varlistentry>

src/backend/utils/misc/guc.c

@@ -4946,7 +4946,7 @@ static struct config_enum ConfigureNamesEnum[] =
 	},
 
 	{
-		{"recovery_init_sync_method", PGC_POSTMASTER, ERROR_HANDLING_OPTIONS,
+		{"recovery_init_sync_method", PGC_SIGHUP, ERROR_HANDLING_OPTIONS,
 			gettext_noop("Sets the method for synchronizing the data directory before crash recovery."),
 		},
 		&recovery_init_sync_method,

src/backend/utils/misc/postgresql.conf.sample

@@ -772,7 +772,6 @@
 					# data?
 					# (change requires restart)
 #recovery_init_sync_method = fsync	# fsync, syncfs (Linux 5.8+)
-					# (change requires restart)
 
 
 #------------------------------------------------------------------------------