Swift: specify treatment for multi-line things

Author: redsun82

swift/ql/src/diagnostics/SuccessfullyExtractedLines.ql

@@ -1,6 +1,6 @@
 /**
  * @name Successfully extracted lines
- * @description Count all lines in source code in which something was extracted.
+ * @description Count all lines in source code in which something was extracted. Entities spanning multiple lines like multi-line strings or comments only contribute one line to this count.
  * @kind metric
  * @id swift/diagnostics/successfully-extracted-lines
  * @tags summary