pwm.c: rename to pwm.cpp and add module documentation

use c++ so that raw string literals can be used

src/systemcmds/pwm/CMakeLists.txt

@@ -37,7 +37,7 @@ px4_add_module(
 	COMPILE_FLAGS
 		-Wno-array-bounds
 	SRCS
-		pwm.c
+		pwm.cpp
 	DEPENDS
 		platforms__common
 	)

src/systemcmds/pwm/pwm.cpp

@@ -32,7 +32,7 @@
  ****************************************************************************/
 
 /**
- * @file pwm.c
+ * @file pwm.cpp
  *
  * PWM servo output configuration and monitoring tool.
  */
@@ -67,7 +67,9 @@
 #include "drivers/drv_pwm_output.h"
 
 static void	usage(const char *reason);
+__BEGIN_DECLS
 __EXPORT int	pwm_main(int argc, char *argv[]);
+__END_DECLS
 
 
 static void
@@ -77,6 +79,34 @@ usage(const char *reason)
 		PX4_WARN("%s", reason);
 	}
 
+	PRINT_MODULE_DESCRIPTION(
+		R"DESCR_STR(
+### Description
+This command is used to configure PWM outputs for servo and ESC control.
+
+The default device `/dev/pwm_output0` are the Main channels, AUX channels are on `/dev/pwm_output1` (`-d` parameter).
+
+It is used in the startup script to make sure the PWM parameters (`PWM_*`) are applied (or the ones provided
+by the airframe config if specified). `pwm info` shows the current settings (the trim value is an offset
+and configured with `PWM_MAIN_TRIMx` and `PWM_AUX_TRIMx`).
+
+The disarmed value should be set such that the motors don't spin (it's also used for the kill switch), at the
+minimum value they should spin.
+
+Channels are assigned to a group. Due to hardware limitations, the update rate can only be set per group. Use
+`pwm info` to display the groups. If the `-c` argument is used, all channels of any included group must be included.
+
+The parameters `-p` and `-r` can be set to a parameter instead of specifying an integer: use -p p:PWM_MIN for example.
+
+### Examples
+Set the PWM rate for all channels to 400 Hz:
+$ pwm rate -a -r 400
+
+Test the outputs of eg. channels 1 and 3, and set the PWM value to 1200 us:
+$ pwm arm
+$ pwm test -c 13 -p 1200
+
+)DESCR_STR");
 
 
 	PRINT_MODULE_USAGE_NAME("pwm", "command");