add prompts

Xcode/DocumentAction.txt

@@ -0,0 +1,18 @@
+The user is curently inside this file: {{filename}}
+The contents are below:
+```swift:{{filename}}
+{{filecontent}}
+```
+
+The user has selected the following code from that file:
+```swift
+{{selected_code}}
+```
+
+The user has asked:
+
+Provide documentation for `{{selected_code}}`.
+
+- Respond with a single code block.
+- Only include documentation comments. No other Swift code.
+