refactor: remove fill_nan method documentation from functions.rst

Author: kosiew

docs/source/user-guide/common-operations/functions.rst

@@ -150,18 +150,3 @@ The ``fill_null()`` method replaces NULL values in specified columns with a prov
     df = df.fill_null("missing", subset=["name", "category"]) 
 
 The fill value will be cast to match each column's type. If casting fails for a column, that column remains unchanged.
-
-fill_nan
---------
-
-The ``fill_nan()`` method replaces NaN values in floating-point columns with a provided numeric value:
-
-.. code-block:: python
-
-    # Fill all NaN values with 0 in numeric columns
-    df = df.fill_nan(0)
-
-    # Fill NaN values in specific numeric columns
-    df = df.fill_nan(99.9, subset=["price", "score"])
-
-This only works on floating-point columns (float32, float64). The fill value must be numeric (int or float).