chore: fixup docstring of TypeMapper.from_string() to be more accurate

Author: NickCrews

ibis/formats/__init__.py

@@ -67,8 +67,7 @@ def from_string(cls, text: str, nullable: bool | None = None) -> DataType:
             The backend-specific string representation to convert.
         nullable
             Whether the Ibis DataType should be nullable.
-            If `None`, the nullability will be inferred from `text` if possible,
-            eg if the string starts with '!' it is considered non-nullable.
+            If `None`, the specific type mapper will choose a default.
 
         Returns
         -------