Document PWM receivers output mapping feature

[gbisaga]

• New paragraph for PWM receivers in the “Other Devices” section: they get Output Mapping and Failsafe Position; using the Output Mapping control opens the output mapping sub-menu.
• Reference to a new Output Mapping image (lua-otherRX-mapping.png; the diff adds .jpg assets, so you may be using PNGs or both).
• Bullet list for the mapping sub-menu: Output Ch, Input Ch, Output Mode, Invert (aligned with the Web UI Connections tab).
• Explanation of Set Failsafe Pos: sets PWM positions on signal loss; Lua can only use current stick positions (unlike the Web UI).
• New Failsafe figure reference (lua-otherRX-failsafe.png).
• Note that Rx Mode (receiver diversity) exists only pre–4.0; in 4.0 it’s negotiated and the field is gone.