docs: specify task.if context

Signed-off-by: Matthias Pichler <[email protected]>

Author: matthias-pichler

dsl-reference.md

@@ -243,7 +243,7 @@ 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. The runtime expression evaluates in the context of the filtered task input (i.e. result of the `input.from` expression, if any) |
 | 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. |