Add a hint to use VisitorState#getSourceForNode in TreeToString.

This is already used in a SuggestedFix if possible, but mentioning it in the description seems potentially useful.

Fixes external #1308

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=258724013

core/src/main/java/com/google/errorprone/bugpatterns/TreeToString.java

@@ -46,8 +46,9 @@
 @BugPattern(
     name = "TreeToString",
     summary =
-        "Tree#toString shouldn't be used for Trees deriving from the code being "
-            + "compiled, as it discards whitespace and comments.",
+        "Tree#toString shouldn't be used for Trees deriving from the code being compiled, as it"
+            + " discards whitespace and comments. If this code is within an ErrorProne check,"
+            + " consider VisitorState#getSourceForNode.",
     severity = WARNING,
     providesFix = NO_FIX)
 public class TreeToString extends AbstractToString {