Copter: ver 3.6 updates

[rmackay9]

Wiki updates we should do before the official Copter-3.6 release:

• SmartRTL page needs info on what happens when it runs out of points (answer: vehicle can't be switched into SmartRTL). Also mention SmartRTL can be used for various failsafes.
• battery failsafe update
• clarify GPS_TYPE must be set specifically to "5" (NMEA) if GPS only provides the NMEA protocol messages
• add dodecahexa copter output to motor mapping
• how to run per-motor compass mot? https://discuss.ardupilot.org/t/implementing-per-motor-calibration/30199
• new loiter tuning parameter info should be added to loiter page
• add info about how compasses are chosen and how to disable some drivers (COMPASS_TYPE_MASK)
• add info about supporting two external compasses
• flowhold mode page
• follow mode page
• ChibiOS-only OSD. http://ardupilot.org/copter/docs/common-osd-boards-on-screen-display.html
• baro temperature calibration. aka TCAL_ENABLED. https://discuss.ardupilot.org/t/tcal-enabled-parameter/34982
• ICE engine integration (https://discuss.ardupilot.org/t/petrol-boosted-tricopter/17823)
• replace references to RC_FEEL_RP with ATC_INPUT_TC on stabilize mode and autotune pages

[al3xst]

Also I guess this chapter has to be adapted to ArduCopter: http://ardupilot.org/dev/docs/code-overview-adding-a-new-mavlink-message.html since commands_logic.cpp and control_auto.cpp don't exist. (restructured? content moved?)

[rmackay9]

Moved remaining issues to the Copter-4.0 wiki list: #2110