Move register_on_modchannel_signal() (#6497)

Moved from lua_api.txt to client_lua_api.md.

doc/client_lua_api.md

@@ -688,7 +688,16 @@ Call these functions only at load time!
    * You must have joined some channels before, and server must acknowledge the
      join request.
    * If message comes from a server mod, `sender` field is an empty string.

* `minetest.register_on_modchannel_signal(func(channel_name, signal))`
    * Called when a valid incoming mod channel signal is received
    * Signal id permit to react to server mod channel events
    * Possible values are:
      0: join_ok
      1: join_failed
      2: leave_ok
      3: leave_failed
      4: event_on_not_joined_channel
      5: state_changed
* `minetest.register_on_inventory_open(func(inventory))`
    * Called when the local player open inventory
    * Newest functions are called first

doc/lua_api.txt

@@ -2529,16 +2529,6 @@ Call these functions only at load time!
    * Called when an incoming mod channel message is received
    * You should have joined  some channels to receive events.
    * If message comes from a server mod, `sender` field is an empty string.
* `minetest.register_on_modchannel_signal(func(channel_name, signal))`
    * Called when a valid incoming mod channel signal is received
    * Signal id permit to react to server mod channel events
    * Possible values are:
      0: join_ok
      1: join_failed
      2: leave_ok
      3: leave_failed
      4: event_on_not_joined_channel
      5: state_changed

### Other registration functions
* `minetest.register_chatcommand(cmd, chatcommand definition)`