Update Doc2Vec documentation: how tags are assigned in `corpus_file…

…` mode (#2320)

* add clarification regarding tags of documents in corpus_file mode for Doc2Vec

* based on -> equal to

gensim/models/doc2vec.py

@@ -487,7 +487,8 @@ def __init__(self, documents=None, corpus_file=None, dm_mean=None, dm=1, dbow_wo
         corpus_file : str, optional
             Path to a corpus file in :class:`~gensim.models.word2vec.LineSentence` format.
             You may use this argument instead of `sentences` to get performance boost. Only one of `sentences` or
-            `corpus_file` arguments need to be passed (or none of them).
+            `corpus_file` arguments need to be passed (or none of them). Documents' tags are assigned automatically
+            and are equal to line number, as in :class:`~gensim.models.doc2vec.TaggedLineDocument`.
         dm : {1,0}, optional
             Defines the training algorithm. If `dm=1`, 'distributed memory' (PV-DM) is used.
             Otherwise, `distributed bag of words` (PV-DBOW) is employed.
@@ -761,7 +762,8 @@ def train(self, documents=None, corpus_file=None, total_examples=None, total_wor
         corpus_file : str, optional
             Path to a corpus file in :class:`~gensim.models.word2vec.LineSentence` format.
             You may use this argument instead of `sentences` to get performance boost. Only one of `sentences` or
-            `corpus_file` arguments need to be passed (not both of them).
+            `corpus_file` arguments need to be passed (not both of them). Documents' tags are assigned automatically
+            and are equal to line number, as in :class:`~gensim.models.doc2vec.TaggedLineDocument`.
         total_examples : int, optional
             Count of sentences.
         total_words : int, optional
@@ -1140,7 +1142,8 @@ def build_vocab(self, documents=None, corpus_file=None, update=False, progress_p
         corpus_file : str, optional
             Path to a corpus file in :class:`~gensim.models.word2vec.LineSentence` format.
             You may use this argument instead of `sentences` to get performance boost. Only one of `sentences` or
-            `corpus_file` arguments need to be passed (not both of them).
+            `corpus_file` arguments need to be passed (not both of them). Documents' tags are assigned automatically
+            and are equal to a line number, as in :class:`~gensim.models.doc2vec.TaggedLineDocument`.
         update : bool
             If true, the new words in `sentences` will be added to model's vocab.
         progress_per : int