text: Title: Wrong power base in LDA Model log_perplexity documentation
  Body: #### Problem description
  
  Gensim LDAModel documentation incorrect
  
  #### Steps/code/corpus to reproduce
  
  Based on the code in log_perplexity, it looks like it should be e^(-bound) since all of the functions used in computing it seem to be using the natural logarithm/e
  
  