Improve comment

Author: gibachan

Sources/SwiftBasicFormat/BasicFormat.swift

@@ -413,8 +413,15 @@ open class BasicFormat: SyntaxRewriter {
       anchorPoints[token] = currentIndentationLevel
     }
 
-    // Add an indentation in the line for the initial token,
-    // if the token is present and its leadingTrivia does not contain the indentation.
+    // If we are formatting a subtree and the line that the initial token occurs on is indented,
+    // use that line indentation for the first token in the subtree to format.
+    // For example, when formatting only the code block in the following,
+    // then the opening `{` should be indented by four spaces.
+    // ```
+    //     func test() {
+    //         print(1)
+    //     }
+    // ``
     if isInitialToken && token.presence == .present {
       let indentationOfLine = token.indentationOfLine
       if token.leadingTrivia.pieces.suffix(indentationOfLine.pieces.count) != indentationOfLine.pieces {