DOC: update information of parameter value_name of pandas.melt
- [x] closes #54715 
- [x] *Pre-commit* and *python3 scripts/validate_docstrings.py pandas.melt* were executed with no mistakes

The update developed was to add the information of no repeating an already existing column name.




@myles could you take a look in this PR, please? I know it's a small thing, but I am new in the open source community then I started with something simple. Thanks @thiagogquinto 