docs(config): describe fallback migration directly

The migration docstring narrated the former key design instead of explaining
the live compatibility path. State what each spelling stores, which value
wins, and when compatibility spellings are removed.

Verified with both full suites, the strict prose checks, and diff checks.

Author: CrazyCoder

src/crossglyph/fontconf.py

@@ -861,12 +861,11 @@ def _space_widths(values: dict[str, str], where: str) -> dict[int, float]:
 
 def _migrate_legacy_fallbacks(path: pathlib.Path,
                               values: dict[str, str]) -> dict[str, str]:
-    """Rename the two family fallback keys in place, once.
+    """Replace compatibility spellings with the family fallback keys.
 
-    The config used to name the regular source file that identifies each
-    fallback family. The build has always resolved the rest of that family
-    from the file. Keep a new key when both spellings are present, and remove
-    the stale spelling either way.
+    Both spellings store the regular source file that identifies a fallback
+    family. Keep the current key when both are present, and remove each
+    compatibility spelling after reading it.
     """
     migrated = dict(values)
     changes: dict[str, str | None] = {}