Refine docstring and simplify reverse_letters implementation (#14205)

* docs: refine docstring and simplify reverse_letters implementation

Updated the docstring for clarity and improved the logic for reversing words.

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Update reverse_letters.py

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Maxim Smolskiy <mithridatus@mail.ru>