DOC: update information of parameter value_name of pandas.melt (#54828)

* doc: updating explanation of parameter value_name to prevent error

* update wording for clarity

* fix spell

* update with the suggested change

Author: thiagogquinto

pandas/core/shared_docs.py

@@ -213,7 +213,7 @@
     Name to use for the 'variable' column. If None it uses
     ``frame.columns.name`` or 'variable'.
 value_name : scalar, default 'value'
-    Name to use for the 'value' column.
+    Name to use for the 'value' column, can't be an existing column label.
 col_level : int or str, optional
     If columns are a MultiIndex then use this level to melt.
 ignore_index : bool, default True