Update docs doubly_linked_list.cpp

[ ] Added description of change
[ ] Added file name matches File name guidelines
[ ] Added tests and example, test must pass
[ ] Added documentation so that the program is self-explanatory and educational - Doxygen guidelines
[ ] Relevant documentation/comments is changed or added
[ ] PR title follows semantic commit guidelines
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[ ] I acknowledge that all my contributions will be made under the project's license.

Notes:

TheAlgorithms / C-Plus-Plus