DOC update DataFrame.to_csv write modes (#51839)

pandas/core/generic.py

@@ -3635,9 +3635,14 @@ def to_csv(
             sequence should be given if the object uses MultiIndex. If
             False do not print fields for index names. Use index_label=False
             for easier importing in R.
-        mode : str, default 'w'
-            Python write mode. The available write modes are the same as
-            :py:func:`open`.
+        mode : {{'w', 'x', 'a'}}, default 'w'
+            Forwarded to either `open(mode=)` or `fsspec.open(mode=)` to control
+            the file opening. Typical values include:
+
+            - 'w', open for writing, truncating the file first.
+            - 'x', open for exclusive creation, failing if the file already exists.
+            - 'a', open for writing, appending to the end of file if it exists.
+
         encoding : str, optional
             A string representing the encoding to use in the output file,
             defaults to 'utf-8'. `encoding` is not supported if `path_or_buf`