Update Documentation related to M204

Documentation/GCodes.md

@@ -66,7 +66,7 @@
 *  M201 - Set max acceleration in units/s^2 for print moves (M201 X1000 Y1000)
 *  M202 - Set max acceleration in units/s^2 for travel moves (M202 X1000 Y1000) Unused in Marlin!!
 *  M203 - Set maximum feedrate that your machine can sustain (M203 X200 Y200 Z300 E10000) in mm/sec
-*  M204 - Set default acceleration: S normal moves T filament only moves (M204 S3000 T7000) im mm/sec^2  also sets minimum segment time in ms (B20000) to prevent buffer underruns and M20 minimum feedrate
+*  M204 - Set default acceleration: P for Printing moves, R for Retract only (no X, Y, Z) moves and T for Travel (non printing) moves (ex. M204 P800 T3000 R9000) in mm/sec^2
 *  M205 - advanced settings:  minimum travel speed S=while printing T=travel only,  B=minimum segment time X= maximum xy jerk, Z=maximum Z jerk, E=maximum E jerk
 *  M206 - set additional homing offset
 *  M207 - set retract length S[positive mm] F[feedrate mm/min] Z[additional zlift/hop], stays in mm regardless of M200 setting

Marlin/Marlin_main.cpp

@@ -143,7 +143,7 @@
 // M201 - Set max acceleration in units/s^2 for print moves (M201 X1000 Y1000)
 // M202 - Set max acceleration in units/s^2 for travel moves (M202 X1000 Y1000) Unused in Marlin!!
 // M203 - Set maximum feedrate that your machine can sustain (M203 X200 Y200 Z300 E10000) in mm/sec
-// M204 - Set default acceleration: S normal moves T filament only moves (M204 S3000 T7000) in mm/sec^2  also sets minimum segment time in ms (B20000) to prevent buffer under-runs and M20 minimum feedrate
+// M204 - Set default acceleration: P for Printing moves, R for Retract only (no X, Y, Z) moves and T for Travel (non printing) moves (ex. M204 P800 T3000 R9000) in mm/sec^2
 // M205 -  advanced settings:  minimum travel speed S=while printing T=travel only,  B=minimum segment time X= maximum xy jerk, Z=maximum Z jerk, E=maximum E jerk
 // M206 - Set additional homing offset
 // M207 - Set retract length S[positive mm] F[feedrate mm/min] Z[additional zlift/hop], stays in mm regardless of M200 setting