text: Title: Update TSNE docs to clarify what is passed in on instantiation or fit
  Body: **Describe the bug**
  Currently, the TSNE docs [show an example](http://www.scikit-yb.org/en/latest/api/text/tsne.html#t-sne-corpus-visualization) where a variable named "labels" is passed into the `fit` method; however this variable actually stores `y` (the target), not the labels (which the `fit` method can derive from `y`). This has [lead to some confusion](https://github.com/DistrictDataLabs/yellowbrick/pull/659) about what values should and can be passed in when and where.
  
  **Proposed Fix**
  Update the TSNE docs to something like:
  
  ```python
  from yellowbrick.text import TSNEVisualizer
  from sklearn.feature_extraction.text import TfidfVectorizer
  
  # Load the data and create document vectors
  corpus = load_corpus('hobbies')
  tfidf  = TfidfVectorizer()
  
  X = tfidf.fit_transform(corpus.data)
  y = corpus.target
  
  tsne = TSNEVisualizer()
  tsne.fit(X, y)
  tsne.poof()
  ```
  
  We should also add another example to illustrate how a user can pass in the labels as a list on instantiation, e.g.:
  
  ```python
  ...
  
  labels = corpus.categories
  
  tsne = TSNEVisualizer(labels=labels)
  tsne.fit(X, y)
  tsne.poof()
  ```
  
  Sincere thanks to @jeromemassot for alerting us to this point of confusion!
  