Improve documentation of data_sync_retry

Reflecting an updated parameter value requires a server restart, which
was not mentioned in the documentation and in postgresql.conf.sample.

Reported-by: Thomas Poty
Discussion: https://postgr.es/m/15659-0cd812f13027a2d8@postgresql.org

doc/src/sgml/config.sgml

@@ -8791,7 +8791,8 @@ dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
        <para>
         When set to off, which is the default, <productname>PostgreSQL</productname>
         will raise a PANIC-level error on failure to flush modified data files
-        to the filesystem.  This causes the database server to crash.
+        to the filesystem.  This causes the database server to crash.  This
+        parameter can only be set at server start.
        </para>
        <para>
         On some operating systems, the status of data in the kernel's page

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

@@ -719,7 +719,9 @@
 
 #exit_on_error = off			# terminate session on any error?
 #restart_after_crash = on		# reinitialize after backend crash?
-#data_sync_retry = off			# retry or panic on failure to fsync data?
+#data_sync_retry = off			# retry or panic on failure to fsync
+					# data?
+					# (change requires restart)
 
 
 #------------------------------------------------------------------------------