Clang: Add commentary about column convertion

... to/from utf8 byte offset used by Clang.

Change-Id: I294d6cd61b416e5f2d64206ee2f3f1b4a91fb1d3
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>

src/plugins/clangcodemodel/clangdiagnosticmanager.cpp

@@ -64,6 +64,9 @@ int positionInText(QTextDocument *textDocument,
 {
     auto textBlock = textDocument->findBlockByNumber(
                 static_cast<int>(sourceLocationContainer.line()) - 1);
+    // 'sourceLocationContainer' already has the CppEditor column converted from
+    // the utf8 byte offset from the beginning of the line provided by clang.
+    // - 1 is required for 0-based columns.
     const int column = static_cast<int>(sourceLocationContainer.column()) - 1;
     return textBlock.position() + column;
 }