fix(python): match multi_word_search solution arg with starter code

[benluo-maui]

The starter code template for multi_word_search in the "Strings and Dictionaries" exercise notebook (notebooks/python/raw/ex_6.ipynb) defines the function signature as:
def multi_word_search(doc_list, keywords):

However, the official solution in learntools/python/solns/multi_word_search.py defines it using documents as the first argument name:
def multi_word_search(documents, keywords):

This mismatch (using documents instead of doc_list in the solution) causes consistency issues and can lead to NameError for learners who try to adapt their starter code using the solution's body.

This PR updates the solution to use doc_list to match the starter code and maintain consistency with Question 2 (word_search), which also uses doc_list.

[benluo-maui]

@jplotts Please take a look.

[jplotts]

Thanks!