Java Style Guide: Call out that stateless lambdas are special.

Change-Id: I94c4c9fb89e6e33f2f87345218607a1ea772d800
Reviewed-on: https://chromium-review.googlesource.com/1212564
Commit-Queue: agrieve <agrieve@chromium.org>
Reviewed-by: Ted Choc <tedchoc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589572}

styleguide/java/java.md

@@ -30,6 +30,9 @@ is encouraged, but there are some gotchas:
 
 ### Lambdas and Method References
  * These are syntactic sugar for creating anonymous inner classes.
+   * Furthermore, stateless lambdas 
+     [become singletons](https://stackoverflow.com/questions/27524445/does-a-lambda-expression-create-an-object-on-the-heap-every-time-its-executed)
+     and so do not result in new instances when used in loops.
  * Use them only where the cost of an extra class & method definition is
    justified.