Fix documentation for gensim.corpora.dictionary and `gensim.corpora…

….hashdictionary`. Partial fix #1671 (#1814)

* Refactored Dictionary description

* Some additions to dictionary.py

* More docstrings

* Docstringed dictionary.py (except from_corpus method)

* Added docstrings for hashdictionary.py

* Fix for numbered lists

* Merged branch

* fix hashdictionary[1]

* fix hashdictionary[2]

* fix hashdictionary[3]

* fix dictionary[1]

* fix hashdictionary[4]

* fix dictionary[2]

* fix dictionary[3]