Clarify handling of filtered terminals

Fix DeprecationWarning in lalr_analysis.py

As discussed in issue #378, when an embedded transformer (that is, one
passed to the Lark class using the transformer argument), is an
inplace transformer (either a subclass of Transformer_InPlace, or with
the @v_args(tree=True) decorator), the in-place transformer was not
working correctly and in-fact Lark used it like a normal non-in-place
transformer, expecting it to return the transformed value.