User's Guide: clarify jtag_rclk advice

Not all cores and boards support adaptive clocking, so qualify
all advice to use it to depend on core and board support.

It's primarily ARM cores which support this; and many of the
newer ones (like Cortex-M series) don't.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>

doc/openocd.texi

@@ -1447,7 +1447,8 @@ Adaptive clocking provides a partial workaround, but a more complete
 solution just avoids using that instruction with JTAG debuggers.
 @end quotation
 
-If the board supports adaptive clocking, use the @command{jtag_rclk}
+If both the chip and the board support adaptive clocking,
+use the @command{jtag_rclk}
 command, in case your board is used with JTAG adapter which
 also supports it.  Otherwise use @command{jtag_khz}.
 Set the slow rate at the beginning of the reset sequence,
@@ -2387,7 +2388,8 @@ However, it introduces delays to synchronize clocks; so it
 may not be the fastest solution.
 
 @b{NOTE:} Script writers should consider using @command{jtag_rclk}
-instead of @command{jtag_khz}.
+instead of @command{jtag_khz}, but only for (ARM) cores and boards
+which support adaptive clocking.
 
 @deffn {Command} jtag_khz max_speed_kHz
 A non-zero speed is in KHZ. Hence: 3000 is 3mhz.