Update error message and documentation

docs/sections/user_guide/cli/tools/mode_config.rst

@@ -434,16 +434,16 @@ and an additional supplemental YAML file ``values2.yaml`` with the following con
 .. note:: In recognition of the different sets of value types representable in each config format, ``uw`` supports two format-combination schemes:
 
    1. **Output matches input:** The format of the output config matches that of the input config.
-   2. **Input is YAML:** If the input config is YAML, any output format may be requested. In the worst case, values always have a string representation, but note that, for example, the string representation of a YAML sequence (Python ``list``) in an INI output config may not be useful.
+   2. **YAML:** YAML is accepted as either input or output with any other format. In the worst case, values always have a string representation, but note that, for example, the string representation of a YAML sequence (Python ``list``) in an INI output config may not be useful.
 
    In all cases, any supplemental configs must be in the same format as the input config and must have recognized extensions.
 
    ``uw`` considers invalid combination requests errors:
 
    .. code-block:: text
 
-      $ uw config realize --input-file b.nml --output-file a.yaml
-      Output format yaml must match input format nml
+      $ uw config realize --input-file b.nml --output-file a.ini
+      Accepted output formats for input format nml are nml or yaml
 
    .. code-block:: text
 

src/uwtools/config/tools.py

@@ -253,8 +253,7 @@ def _validate_format_output(input_fmt: str, output_fmt: str) -> None:
     """
     if FORMAT.yaml not in (input_fmt, output_fmt) and input_fmt != output_fmt:
         raise UWError(
-            "Output format %s must match input format %s unless either is YAML"
-            % (output_fmt, input_fmt)
+            "Accepted output formats for input format %s are %s or yaml" % (input_fmt, input_fmt)
         )
 
 

src/uwtools/tests/config/test_tools.py

@@ -651,8 +651,8 @@ def test__validate_format_output(input_fmt, output_fmt):
         with raises(UWError) as e:
             call()
         assert (
-            str(e.value) == f"Output format {output_fmt} must match input format {input_fmt}"
-            " unless either is YAML"
+            str(e.value) == "Accepted output formats for input format "
+            f"{input_fmt} are {input_fmt} or yaml"
         )