feat: add ACTOR_RESTART_ON_ERROR environment variable docs

Author: DaveHanns

sources/platform/actors/development/programming_interface/environment_variables.md

@@ -50,6 +50,7 @@ Here's a table of key system environment variables:
 | `ACTOR_INPUT_KEY` | Key of the record in the default key-value store that holds the [Actor input](/platform/actors/running/input-and-output#input).  |
 | `ACTOR_MAX_PAID_DATASET_ITEMS` | For paid-per-result Actors, the user-set limit on returned results. Do not exceed this limit. |
 | `ACTOR_MAX_TOTAL_CHARGE_USD` | For pay-per-event Actors, the user-set limit on run cost. Do not exceed this limit. |
+| `ACTOR_RESTART_ON_ERROR` | If **1**, the Actor run will be restarted if it fails. |
 | `APIFY_HEADLESS` | If **1**, web browsers inside the Actor should run in headless mode (no windowing system available). |
 | `APIFY_IS_AT_HOME` | Contains **1** if the Actor is running on Apify servers. |
 | `ACTOR_MEMORY_MBYTES` | Size of memory allocated for the Actor run, in megabytes. Can be used to optimize memory usage or finetuning of low-level external libraries. |