add javadoc to a method in core/test/com/google/inject/spi/BindingSourceRestrictionTest.java

Hi there, when I was reading the code of the function "void assertThatInjectorIsWiped(Injector injector)" in core/test/com/google/inject/spi/BindingSourceRestrictionTest.java, I found it has no Javadoc. I would like to contribute by adding “this method is used to assert That Injector Is Wiped Injector". Can you please check if this is correct? Thanks.

Open core/test/com/google/inject/spi/BindingSourceRestrictionTest.java and read assertThatInjectorIsWiped(Injector injector) in context. Add concise Javadoc that accurately describes the method's assertion, then verify the file's tests still pass.

Written by the indexing model from the issue text.