Add warning when string is used as argument to infer_vector() (#2347)

* correct name of 'sentences' argument to 'documents in docstrings; minor docstring clarifications over which I stumbled

* add warning when string is used as argument to infer_vector()

* change warning about single string to exception

* change ValueError to TypeError and make error message more explicit

gensim/models/doc2vec.py

@@ -907,6 +907,9 @@ def infer_vector(self, doc_words, alpha=None, min_alpha=None, epochs=None, steps
             The inferred paragraph vector for the new document.
 
         """
+        if isinstance(doc_words, string_types):
+            raise TypeError("Parameter doc_words of infer_vector() must be a list of strings (not a single string).")
+
         alpha = alpha or self.alpha
         min_alpha = min_alpha or self.min_alpha
         epochs = epochs or steps or self.epochs