Add documentation of all agent configuration options (#667)

Co-authored-by: Anbraten <anton@ju60.de>
Co-authored-by: 6543 <6543@obermui.de>

docs/docs/30-administration/11-agent-config.md

@@ -95,20 +95,72 @@ See [Conditionals Pipeline](/docs/usage/pipeline-syntax#step-when---conditional-
 
 Here is the full list of configuration options and their default variables. 
 
-```yaml
-    - WOODPECKER_SERVER=localhost:9000
-    - WOODPECKER_AGENT_SECRET=""
-    - WOODPECKER_USERNAME=x-oauth-basic
-    - WOODPECKER_LOG_LEVEL=""
-    - WOODPECKER_DEBUG_PRETTY=""
-    - WOODPECKER_DEBUG_NOCOLOR=true
-    - WOODPECKER_HOSTNAME=""
-    - WOODPECKER_PLATFORM="linux/amd64"
-    - WOODPECKER_FILTER=""
-    - WOODPECKER_MAX_PROCS=1
-    - WOODPECKER_HEALTHCHECK=true
-    - WOODPECKER_KEEPALIVE_TIME=10
-    - WOODPECKER_KEEPALIVE_TIMEOUT=time.Second * 20
-    - WOODPECKER_GRPC_SECURE=""
-    - WOODPECKER_GRPC_VERIFY=true
-```
+### `WOODPECKER_SERVER`
+> Default: `localhost:9000`
+
+Configures gRPC address of the server.
+
+### `WOODPECKER_USERNAME`
+> Default: `x-oauth-basic`
+
+The gRPC username.
+
+### `WOODPECKER_AGENT_SECRET`
+> Default: empty
+
+A shared secret used by server and agents to authenticate communication. A secret can be generated by `openssl rand -hex 32`.
+
+### `WOODPECKER_LOG_LEVEL`
+> Default: empty
+
+Configures the logging level. Possible values are `trace`, `debug`, `info`, `warn`, `error`, `fatal`, `panic`, `disabled` and empty.
+
+### `WOODPECKER_DEBUG_PRETTY`
+> Default: `false`
+
+Enable pretty-printed debug output.
+
+### `WOODPECKER_DEBUG_NOCOLOR`
+> Default: `true`
+
+Disable colored debug output.
+
+### `WOODPECKER_HOSTNAME`
+> Default: empty
+
+Configures the agent hostname.
+
+### `WOODPECKER_MAX_PROCS`
+> Default: `1`
+
+Configures the number of parallel builds.
+
+### `WOODPECKER_HEALTHCHECK`
+> Default: `true`
+
+Enable healthcheck endpoint.
+
+### `WOODPECKER_KEEPALIVE_TIME`
+> Default: empty
+
+After a duration of this time of no activity, the agent pings the server to check if the transport is still alive.
+
+### `WOODPECKER_KEEPALIVE_TIMEOUT`
+> Default: `20s`
+
+After pinging for a keepalive check, the agent waits for a duration of this time before closing the connection if no activity.
+
+### `WOODPECKER_GRPC_SECURE`
+> Default: `false`
+
+Configures if the connection to `WOODPECKER_SERVER` should be made using a secure transport.
+
+### `WOODPECKER_GRPC_VERIFY`
+> Default: `true`
+
+Configures if the gRPC server certificate should be verified, only valid when `WOODPECKER_GRPC_SECURE` is `true`.
+
+### `WOODPECKER_BACKEND`
+> Default: `auto-detect`
+
+Configures the backend engine to run pipelines on. Possible values are `auto-detect` or `docker`.