docs(data-modeling,skills): re-quote the corrected searchable-fields hint, drop the moot caveat

Fixes #6925

PR #6921 (#6673) changed the `searchable-field-unknown` lint hint in
`packages/lint/src/validate-searchable-fields.ts` from "copy the value onto a
formula field here" to "copy the value onto a stored text field here" — a
formula field is virtual and materializes no column, so the old prescription
never worked.

Two docs blocks quote that hint verbatim (added by PR #6670, before #6673
landed) and each carried an inline caveat noting the quoted wording was loose.
Both are now stale: the quote reproduces text the tool no longer emits, and
the caveat describes wording that no longer exists.

- content/docs/data-modeling/schema-design.mdx: re-quote + drop caveat
- skills/objectstack-data/SKILL.md: re-quote + drop caveat

Verified the corrected quote is byte-for-byte identical to the tool's actual
hint string on origin/main. No other content/docs/** or skills/** location
quotes the stale "formula field" wording (grepped repo-wide).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01F8q5J1MQyocgtNspb15fSn

Author: claude

content/docs/data-modeling/schema-design.mdx

@@ -169,14 +169,11 @@ every entry is dropped.
 
 hint: 'search' scans this object's own columns, so a related record's column
 cannot be a search target — expand the relation and search the related object,
-or copy the value onto a formula field here. Clients echo this declaration
+or copy the value onto a stored text field here. Clients echo this declaration
 verbatim as the '$searchFields' override, so a stale entry becomes a 400
 INVALID_FIELD on list search (#4254), not just a quietly narrowed one.
 ```
 
-(That hint's "text/formula" family of wording is loose — only the **stored**
-half works; see the callout above.)
-
 A request that sends the dotted path is `400 INVALID_FIELD`:
 
 ```text

skills/objectstack-data/SKILL.md

@@ -168,13 +168,11 @@ every entry is dropped.
 
 hint: 'search' scans this object's own columns, so a related record's column
 cannot be a search target — expand the relation and search the related object,
-or copy the value onto a formula field here. Clients echo this declaration
+or copy the value onto a stored text field here. Clients echo this declaration
 verbatim as the '$searchFields' override, so a stale entry becomes a 400
 INVALID_FIELD on list search (#4254), not just a quietly narrowed one.
 ```
 
-(The hint's "formula field" wording is loose — only a **stored** mirror works.)
-
 A request carrying the dotted path is `400 INVALID_FIELD`:
 
 ```text