Document table JSON extras from metadata

docs/json_api_doc.py

@@ -0,0 +1,20 @@
+def table_extras(cog):
+    from datasette.extras import ExtraScope
+    from datasette.views.table_extras import table_extra_registry
+
+    cog.out("\n.. list-table::\n")
+    cog.out("   :header-rows: 1\n\n")
+    cog.out("   * - Extra\n")
+    cog.out("     - Description\n")
+    for cls in table_extra_registry.public_classes_for_scope(ExtraScope.TABLE):
+        description = cls.description or ""
+        notes = []
+        if cls.expensive:
+            notes.append("May execute additional queries.")
+        if cls.docs_note:
+            notes.append(cls.docs_note)
+        if notes:
+            description = "{} ({})".format(description, " ".join(notes)).strip()
+        cog.out("   * - ``{}``\n".format(cls.key()))
+        cog.out("     - {}\n".format(description))
+    cog.out("\n")