Fix help text for stevedore_namespace BUILD file symbol.

[kaos]

Closes #20977

❯ pants --backend-packages=pants.backend.experimental.python.framework.stevedore stevedore_namespace --help
`stevedore_namespace` BUILD file symbol
---------------------------------------

Tag a namespace in entry_points as a stevedore namespace.

This is required for the entry_point to be visible to dep inference
based on the `stevedore_namespaces` field.

For example:
    python_distribution(
        ...
        entry_points={{
            stevedore_namespace("a.b.c"): {{
                "plugin_name": "some.entry:point",
            }},
        }},
    )

[kaos]

TIL: f-strings don't work as __doc__ strings. (I may have known this in the past, but re-discovered it today..)

[huonw]

Oooh, subtle. Good catch.

[huonw]

Nice one!