docs(plugins): correct before_run_callback flow control behavior

The "Runner start callbacks" section described `before_run_callback` as
returning a `types.Content` to "replace the user's original message". That is
actually the behavior of `on_user_message_callback`; the text (including the
"Purpose" line) was duplicated from that section.

In the implementation, returning a `Content` from `before_run_callback` halts
the run: the Runner yields it as an early-exit event and skips all subsequent
processing instead of invoking the agent.

See `Runner._exec_with_plugin` in `src/google/adk/runners.py`:

    early_exit_result = await plugin_manager.run_before_run_callback(...)
    if isinstance(early_exit_result, types.Content):
        early_exit_event = Event(..., content=early_exit_result)
        ...
        yield early_exit_event      # early exit, agent never runs
    else:
        # normal execution

This also matches the BasePlugin docstring: "Returning a value to halt
execution of the runner and ends the runner with that event."

Fix the "When It Runs", "Purpose", and "Flow Control" bullets accordingly.

Author: yana-gi

docs/plugins/index.md

@@ -796,12 +796,12 @@ object takes the potentially modified user message and prepares for execution.
 The `before_run_callback` fires here, allowing for global setup before any agent
 logic begins.
 
--   **When It Runs:** Immediately after `runner.run()` is called, before
-    any other processing.
--   **Purpose:** The first opportunity to inspect or modify the user's raw
-    input.
--   **Flow Control:** Return a `types.Content` object to **replace** the
-    user's original message.
+-   **When It Runs:** After the `on_user_message_callback`, when the `Runner`
+    prepares for execution and before any agent logic begins.
+-   **Purpose:** Global setup or initialization before the invocation runs.
+-   **Flow Control:** Return a `types.Content` object to **halt execution**:
+    the `Runner` exits early and ends the run with that content as the result.
+    Return `None` to proceed normally.
 
 The following code example shows the basic syntax of this callback: