Updated the description of the telemetry script background function, which changed in OpenTX 2.1 (opentx/opentx#4016)

Author: projectkk2glider

telemetry.md

@@ -13,7 +13,9 @@ Scripts are located on the SD card in the folder /SCRIPTS/TELEMETRY/<*name*&g
 Telemetry scripts are started when the model is loaded.
 
 * script init function is called
-* script background function is periodically called when custom telemetry screen is **not visible**
+* script background function is periodically called when custom telemetry screen is **not visible**. *Notice*:
+  * In OpenTX 2.0 this function is **not called** when the custom telemetry screen is visible.
+  * In OpenTX 2.1 and successors this function is **always called** no matter if the custom screen is visible or not.
 * script run function is periodically called when custom telemetry screen is **visible**
 * script is stopped and disabled if it misbehaves (too long runtime, error in code, low memory)
 * all telemetry scripts are stopped while one-time script is running (see Lua One-time scripts)