stm32f2xxx: comments about frequency choice

jimtcl @ 60dfb023

-Subproject commit 411e92fea9621630eb350e0c2bb43543e553b84f
+Subproject commit 60dfb023c4afa95047e0fa8db49830ccb46446b2

tcl/target/stm32f2xxx.cfg

@@ -21,6 +21,11 @@ if { [info exists WORKAREASIZE] } {
 }
 
 # JTAG speed should be <= F_CPU/6. F_CPU after reset is 8MHz, so use F_JTAG = 1MHz
+#
+# Since we may be running of an RC oscilator, we crank down the speed a
+# bit more to be on the safe side. Perhaps superstition, but if are
+# running off a crystal, we can run closer to the limit. Note
+# that there can be a pretty wide band where things are more or less stable.
 jtag_khz 1000
 
 jtag_nsrst_delay 100