Add ctor comment

--
MOS_MIGRATED_REVID=106410161
diff --git a/src/main/java/com/google/devtools/build/lib/analysis/Runfiles.java b/src/main/java/com/google/devtools/build/lib/analysis/Runfiles.java
index 532b929..2540839 100644
--- a/src/main/java/com/google/devtools/build/lib/analysis/Runfiles.java
+++ b/src/main/java/com/google/devtools/build/lib/analysis/Runfiles.java
@@ -508,8 +508,12 @@
       this.suffix = "";
     }
 
-    public Builder(String suffix) {
-      this.suffix = suffix;
+    /**
+     * Creates a builder with the given suffix.
+     * @param workspace is the string specified in workspace() in the WORKSPACE file.
+     */
+    public Builder(String workspace) {
+      this.suffix = workspace;
     }
 
     /**