diff --git a/CHANGES b/CHANGES
index 1c53c1b..b2e944a 100644
--- a/CHANGES
+++ b/CHANGES
@@ -32,6 +32,14 @@ $ uvx --from 'gp-libs' --prerelease allow gp-libs
 _Notes on upcoming releases will be added here_
 <!-- END PLACEHOLDER - ADD NEW CHANGELOG ENTRIES BELOW THIS LINE -->
 
+### Documentation
+
+#### Extension metadata fields are described (#81)
+
+The `TypedDict` returned from `linkify_issues`'s Sphinx `setup()` now says
+what each key holds. They previously reached the rendered API reference as
+"Alias for field number 0" or as a bare name carrying only its type.
+
 ### Development
 
 #### CI actions updated to current majors
diff --git a/src/linkify_issues.py b/src/linkify_issues.py
index 0d0c0db..ee935d7 100644
--- a/src/linkify_issues.py
+++ b/src/linkify_issues.py
@@ -68,7 +68,20 @@ def condition(node: nodes.Node) -> bool:
 
 
 class SetupDict(t.TypedDict):
-    """Setup mapping for Sphinx app."""
+    """Setup mapping for Sphinx app.
+
+    Attributes
+    ----------
+    version : str
+        Version of this extension, returned to Sphinx so it can invalidate
+        its build environment when the extension changes.
+    parallel_read_safe : bool
+        Whether Sphinx may read source files in parallel while this
+        extension is loaded.
+    parallel_write_safe : bool
+        Whether Sphinx may write output in parallel while this extension is
+        loaded.
+    """
 
     version: str
     parallel_read_safe: bool