Dictionary doc: ref FAQ entry about filter_extremes corpus migration (p…

…iskvorky#3257)

gensim/corpora/dictionary.py

@@ -330,7 +330,9 @@ def filter_extremes(self, no_below=5, no_above=0.5, keep_n=100000, keep_tokens=N
 
  After the pruning, resulting gaps in word ids are shrunk.
  Due to this gap shrinking, **the same word may have a different word id before and after the call
- to this function!**
+ to this function!** See :class:`gensim.models.VocabTransform` and the
+ `dedicated FAQ entry <https://github.com/RaRe-Technologies/gensim/wiki/Recipes-&-FAQ#q8-how-can-i-filter-a-saved-corpus-and-its-corresponding-dictionary>`_ on how
+ to transform a corpus built with a dictionary before pruning.
 
  Examples
  --------