docs: add retry_max agent config option (#15487)

website/content/docs/agent/config/config-files.mdx

@@ -1503,6 +1503,8 @@ Valid time units are 'ns', 'us' (or 'µs'), 'ms', 's', 'm', 'h'."
 
 - `retry_interval` Equivalent to the [`-retry-interval` command-line flag](/docs/agent/config/cli-flags#_retry_interval).
 
+- `retry_max` - Equivalent to the [`-retry-max`](/docs/agent/config/cli-flags#_retry_max) command-line flag.
+
 - `retry_join_wan` Equivalent to the [`-retry-join-wan` command-line flag](/docs/agent/config/cli-flags#_retry_join_wan). Takes a list of addresses to attempt joining to WAN every [`retry_interval_wan`](#_retry_interval_wan) until at least one join works.
 
 - `retry_interval_wan` Equivalent to the [`-retry-interval-wan` command-line flag](/docs/agent/config/cli-flags#_retry_interval_wan).