docs: clarify content of svelte/transition module reference page (#15296)

The `/docs/svelte/svelte-transition` reference page previously contained
only the auto-generated `MODULE: svelte/transition` line, which left
users who followed the link from the "transition:" guide with no
context for what the module is or where to learn about it.

Add a short overview that names the built-in transitions and points the
reader to the [Transitions guide] for usage examples and parameters.

Fixes #15296

documentation/docs/98-reference/21-svelte-transition.md

@@ -4,3 +4,13 @@ tags: transitions
 ---
 
 > MODULE: svelte/transition
+
+The [`svelte/transition`](svelte-transition) module provides a small set of
+built-in [transition functions](/docs/svelte/transition) such as `fade`,
+`fly`, `slide`, `scale`, `draw`, `blur`, and `crossfade`. They are
+imported into a `.svelte` component and used with the
+[`transition:`](/docs/svelte/transition), [`in:`](/docs/svelte/in-and-out),
+and [`out:`](/docs/svelte/in-and-out) directives.
+
+See the [Transitions guide](/docs/svelte/transition) for usage examples,
+parameter details, and notes on writing custom transitions.