Refactor API Reference for gensim.parsing. Fix #1664 (#1684)

* Added\fixed docstrings for strip_tags in preprocessing.py

* Added docstrings for strip_numeric, strip_non_alphanum & strip_multiple_whitespaces

* small fixes

* Added docstrings for split_alphanum, stem_text, need additional check for preprocess_string & preprocess_documents

* Fix for old stringdocs and even more!

* Additional changes for preprocessing.py and some refactoring for porter.py

* Added references for functions + some common refactoring

* Added annotations for porter.py & preprocessing.py

* Fixes for annotations

* Refactoring for Attributes and Notes fields

* Reduced some extra large docstrings

* porter.py , function _ends : changed return type from (int) to (bool)

* small fix for sections

* Cleanup porter.py

* Resolve last review

* finish with porter, yay!

* Fix preprocessing

* small changes

* Fix review comments

gensim/parsing/__init__.py

@@ -1,8 +1,5 @@
-"""
-This package contains functions to preprocess raw text
-"""
+"""This package contains functions to preprocess raw text"""
 
-# bring model classes directly into package namespace, to save some typing
 from .porter import PorterStemmer  # noqa:F401
 from .preprocessing import (remove_stopwords, strip_punctuation, strip_punctuation2,  # noqa:F401
                             strip_tags, strip_short, strip_numeric,