Add paragraph to the corpora tutorial describing dictionary.dfs and d…

…ictionary.compactify() (#1347)

In code snippet 13 there are two new concepts introduced that have not
been explained yet. In addition the workflow to create the dictionary
here is completely different from the workflow described in code
snippets 4 and 5. I've added a paragraph that tries to explain the new
workflow and concepts.

docs/notebooks/Corpora_and_Vector_Spaces.ipynb

@@ -340,7 +340,7 @@
    "source": [
     "Although the output is the same as for the plain Python list, the corpus is now much more memory friendly, because at most one vector resides in RAM at a time. Your corpus can now be as large as you want.\n",
     "\n",
-    "Similarly, to construct the dictionary without loading all texts into memory:"
+    "We are going to create the dictionary from the mycorpus.txt file without loading the entire file into memory. Then, we will generate the list of token ids to remove from this dictionary by querying the dictionary for the token ids of the stop words, and by querying the document frequencies dictionary (dictionary.dfs) for token ids that only appear once. Finally, we will filter these token ids out of our dictionary and call dictionary.compactify() to remove the gaps in the token id series."
    ]
   },
   {