docs: document context of task.if

Author: matthias-pichler

dsl-reference.md

@@ -249,7 +249,7 @@ The Serverless Workflow DSL defines a list of [tasks](#task) that **must be** su
 
 | Name | Type | Required | Description|
 |:--|:---:|:---:|:---|
-| if | `string` | `no` | A [`runtime expression`](dsl.md#runtime-expressions), if any, used to determine whether or not the task should be run.<br>The task is considered skipped if not run. |
+| if | `string` | `no` | A [`runtime expression`](dsl.md#runtime-expressions), if any, used to determine whether or not the task should be run.<br>The task is considered skipped if not run and the *raw* task input becomes the task's output. The expression is evaluated against the *raw* task input, before any other expression of the task. |
 | input | [`input`](#input) | `no` | An object used to customize the task's input and to document its schema, if any. |
 | output | [`output`](#output) | `no` | An object used to customize the task's output and to document its schema, if any. |
 | export | [`export`](#export) | `no` | An object used to customize the content of the workflow context. | 

dsl.md

@@ -256,6 +256,7 @@ flowchart TD
   workflow_transformed_input{{Transformed Workflow Input}}
 
   task_raw_input{{Raw Task Input}}
+  task_if[Task: <code>if</code>]
   task_input_schema[\Task: <code>input.schema</code>/]
   task_input_from[Task: <code>input.from</code>]
   task_transformed_input{{Transformed Task Input}}
@@ -282,7 +283,8 @@ flowchart TD
 
   subgraph Task
 
-    task_raw_input -- Validated by --> task_input_schema
+    task_raw_input -- Passed to --> task_if
+    task_if -- Validated by --> task_input_schema
     task_input_schema -- Passed to --> task_input_from
     task_input_from -- Produces --> task_transformed_input
     task_transformed_input -- Set as --> input_arg