docs(error-catalog): INVALID_FIELD names the refusal of a search over an empty resolved search set

The engine refuses a non-blank `search` with `400 INVALID_FIELD`
(`param: 'search'`) when the object's resolved search set is empty,
instead of answering every row. The catalog's Cause text listed every
other read axis that answers this code but not this one. One sentence
is added after the `internal: true` sentence, with the two remedies the
refusal message itself prescribes.

Claude-Session: https://claude.ai/code/session_01FM29ZqLTrnPNXxijioZwMp
Co-authored-by: Claude <noreply@anthropic.com>

Author: claude

content/docs/api/error-catalog.mdx

@@ -159,6 +159,10 @@ aggregation's own `filter`), a sort, a `groupBy`, an `aggregations[].field`, a
 `searchFields` entry, or an `expand` entry's own filter or sort at any depth. Such a
 field is never returned on a generic read, so it cannot be queried by either — the
 refusal names the object that declares it. Query by another field.
+A non-blank `search` is refused (`param: 'search'`), not answered with every row,
+when the object's resolved search set is empty: no `searchableFields` entry names a
+stored field, and the default search scans none (`internal: true` fields never
+count). Declare one that does, or mirror a composed title onto a stored text field.
 Off the request path the same code answers `backfillSummaryNulls`'s
 `recomputeUndefinedOnEmpty` (`os migrate summary-nulls
 --recompute-undefined-on-empty object.field`) when an entry is not a roll-up