Improve clarity and CTRL+F-ability of fixed timestep migration advice (#582)

Prompted by a discussion with @laundmo [on Discord](https://discord.com/channels/691052431525675048/742569353878437978/1081968471090405446).

Author: alice-i-cecile

content/learn/book/migration-guides/0.9-0.10/_index.md

@@ -843,7 +843,7 @@ The `FrameCount`  resource was previously only updated when using the `bevy_rend
 
 #### Multiple fixed timesteps
 
-Apps may now only have one unified fixed timestep. If you were relying on this functionality, you should swap to using timers, via the `on_timer(MY_PERIOD)` run condition.
+Apps may now only have one unified fixed timestep. If you were relying on multiple `FixedTimestep` run criteria with distinct periods, you should swap to using timers, via the `on_timer(MY_PERIOD)` or `on_fixed_timer(MY_PERIOD)` run conditions.
 
 ### [Pipelined Rendering](https://github.com/bevyengine/bevy/pull/6503)