Add doc comment to Statement @param line that it's 1-base

Author: tanishiking

compiler/src/dotty/tools/dotc/coverage/Coverage.scala

@@ -11,7 +11,12 @@ class Coverage:
 
   def addStatement(stmt: Statement): Unit = statementsById(stmt.id) = stmt
 
-/** A statement that can be invoked, and thus counted as "covered" by code coverage tools. */
+
+/**
+  * A statement that can be invoked, and thus counted as "covered" by code coverage tools.
+  *
+  * @param line 1-indexed line number
+  */
 case class Statement(
     location: Location,
     id: Int,