"""Extract parameter descriptions from function docstring.

Parses the function's docstring to extract descriptions for each parameter. Looks for lines that start with parameter names followed by descriptions. Parameters without descriptions are omitted.

Args:
    func: The function to extract parameter descriptions from. Must have a docstring.

Returns:
    Dict[str, Optional[str]]: A dictionary mapping parameter names to their descriptions. Only parameters with descriptions are included.

Example:
    For a function with docstring containing "param1: Description of param1", returns {"param1": "Description of param1"}.
"""