learning_curve.py

* DOC framework for keeping API refs for deprecated classes/funcs

* DOC tagging deprecated for 0.20

* suggestion for LDA/QDA deprecation

* simplify deprecation message for GaussianProcess

* simplify deprecation messages

* fixup avoid import QuadraticDiscriminantAnalysis in qda.QDA (and similar for LDA).

* fixup test alias lda.LDA is instance of LinearDiscriminantAnalysis