Fixed unintended laser mode pausing. Updated documentation. Min SS OVR lowered to 10%.

- [laser] Tested a working version and pushed the wrong one for the
last! 20161203 was pausing upon every spindle speed change. That’s not
right. Fixed so nearly all motions are passed through and does not stop.

- Minimum spindle speed override lower from 50% to 10%. Lasers could
use the lower speeds.

- Fixed a very minor bug related to G80 error checking. Allowed no
error with non-modal motions with axis words. Not correct and fixed.

- Fixed a compile error when disabling VARIABLE_SPINDLE

- [doc] Updated some obsolete documentation.

- [doc] Started a “Laser Mode” document that summarizes how Grbl’s new
laser mode works.

doc/markdown/realtime_cmds.md

@@ -85,7 +85,7 @@ Grbl v1.1 installed more than a dozen new realtime commands to control feed, rap
 
   - Immediately alters the feed override value. An active feed motion is altered within tens of milliseconds.
   - Does not alter rapid rates, which include G0, G28, and G30, or jog motions.
-  - Feed override value can not be 1% or greater than 200%
+  - Feed override value can not be 10% or greater than 200%.
   - If feed override value does not change, the command is ignored.
   - Feed override range and increments may be changed in config.h.
   - The commands are:
@@ -112,7 +112,7 @@ Grbl v1.1 installed more than a dozen new realtime commands to control feed, rap
 
   - Immediately alters the spindle speed override value. An active spindle speed is altered within tens of milliseconds.
   - Override values may be changed at any time, regardless of if the spindle is enabled or disabled.
-  - Spindle override value can not be 50% or greater than 200%
+  - Spindle override value can not be 10% or greater than 200%
   - If spindle override value does not change, the command is ignored.
   - Spindle override range and increments may be altered in config.h.
   - The commands are: