[MRG] Documenting char_wb padding functionality (Issue scikit-learn#8694) (scikit-learn#8803)

* Documenting char_wb padding functionality (Issue scikit-learn#8694)

* Small fix: change of wording.

* 's/passed with space/padded with space/g'

Author: edrogers

sklearn/feature_extraction/text.py

@@ -159,7 +159,8 @@ def _char_wb_ngrams(self, text_document):
         """Whitespace sensitive char-n-gram tokenization.
 
         Tokenize text_document into a sequence of character n-grams
-        excluding any whitespace (operating only inside word boundaries)"""
+        operating only inside word boundaries. n-grams at the edges
+        of words are padded with space."""
         # normalize white spaces
         text_document = self._white_spaces.sub(" ", text_document)
 
@@ -354,7 +355,7 @@ class HashingVectorizer(BaseEstimator, VectorizerMixin):
     analyzer : string, {'word', 'char', 'char_wb'} or callable
         Whether the feature should be made of word or character n-grams.
         Option 'char_wb' creates character n-grams only from text inside
-        word boundaries.
+        word boundaries; n-grams at the edges of words are padded with space.
 
         If a callable is passed it is used to extract the sequence of features
         out of the raw, unprocessed input.
@@ -553,7 +554,7 @@ class CountVectorizer(BaseEstimator, VectorizerMixin):
     analyzer : string, {'word', 'char', 'char_wb'} or callable
         Whether the feature should be made of word or character n-grams.
         Option 'char_wb' creates character n-grams only from text inside
-        word boundaries.
+        word boundaries; n-grams at the edges of words are padded with space.
 
         If a callable is passed it is used to extract the sequence of features
         out of the raw, unprocessed input.