target/stm32f7x: clarify reset_config comment

The reset_config line in the config file does not actually set
connect_assert_srst (the default is connect_deassert_srst), but it reads
as if it does. Clarify that the target is compatible with
connect_assert_srst, without suggesting that the file actually sets it
to that value.

Change-Id: I14e9445ab282d386b5d0055f6adf03d7c8878a8c
Signed-off-by: Christopher Head <chead@zaber.com>
Reviewed-on: http://openocd.zylin.com/4743
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>

tcl/target/stm32f7x.cfg

@@ -58,7 +58,10 @@ if {[using_jtag]} {
  jtag_ntrst_delay 100
 }
 
-# use hardware reset, connect under reset
+# Use hardware reset.
+#
+# This target is compatible with connect_assert_srst, which may be set in a
+# board file.
 reset_config srst_only srst_nogate
 
 if {![using_hla]} {