probe: Rework the PROBE_ACCURACY command parameters

Don't default to a Z location of 10, as that could cause damage if the
probe's z_offset is greater than 10.  Instead, use the "retract
distance" method that is used for normal multi-sample probing.

Update the PROBE_ACCURACY command parameter names to use the same
parameter names as the PROBE command.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

docs/Config_Changes.md

@@ -6,6 +6,10 @@ All dates in this document are approximate.
 
 # Changes
 
+20190710: The optional parameters of the PROBE_ACCURACY command have
+changed. It may be necessary to update any macros or scripts that use
+that command.
+
 20190628: All configuration options have been removed from the
 [skew_correction] section.  Configuration for skew_correction
 is now done via the SET_SKEW gcode.  See skew_correction.md

docs/G-Codes.md

@@ -231,10 +231,12 @@ enabled:
   for details).
 - `QUERY_PROBE`: Report the current status of the probe ("triggered"
   or "open").
-- `PROBE_ACCURACY [REPEAT=<times>] [SPEED=<speed mm/s>] [X=<x pos>]
-  [Y=<y pos>] [Z=<z height>]`: Calculate the maximum, minimum, average,
-  median and standard deviation. The default values are: REPEAT=10,
-  SPEED=probe config speed, X=current X, Y=current Y and Z=10.
+- `PROBE_ACCURACY [PROBE_SPEED=<mm/s>] [SAMPLES=<count>]
+  [SAMPLE_RETRACT_DIST=<mm>]`: Calculate the maximum, minimum,
+  average, median, and standard deviation of multiple probe
+  samples. By default, 10 SAMPLES are taken. Otherwise the optional
+  parameters default to their equivalent setting in the probe config
+  section.
 - `PROBE_CALIBRATE [SPEED=<speed>] [<probe_parameter>=<value>]`: Run a
   helper script useful for calibrating the probe's z_offset. See the
   PROBE command for details on the optional probe parameters. See the