add user docs for ssr assist

Author: JoshMcguigan

crates/ide_ssr/src/lib.rs

@@ -57,6 +57,15 @@
 //
 // | VS Code | **Rust Analyzer: Structural Search Replace**
 // |===
+//
+// Also available as an assist, by writing a comment containing the structural
+// search and replace rule. You will only see the assist if the comment can
+// be parsed as a valid structural search and replace rule.
+//
+// ```rust
+// // Place the cursor on the line below to see the assist 💡.
+// // foo($a, $b) ==>> ($a).foo($b)
+// ```
 
 mod matching;
 mod nester;