doc: clarify sqlite user-defined function behaviour

PR-URL: #56786
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Ulises Gascón <[email protected]>

doc/api/sqlite.md

@@ -176,11 +176,14 @@ added: v23.5.0
   * `useBigIntArguments` {boolean} If `true`, integer arguments to `function`
     are converted to `BigInt`s. If `false`, integer arguments are passed as
     JavaScript numbers. **Default:** `false`.
-  * `varargs` {boolean} If `true`, `function` can accept a variable number of
-    arguments. If `false`, `function` must be invoked with exactly
-    `function.length` arguments. **Default:** `false`.
+  * `varargs` {boolean} If `true`, `function` may be invoked with any number of
+    arguments (between zero and [`SQLITE_MAX_FUNCTION_ARG`][]). If `false`,
+    `function` must be invoked with exactly `function.length` arguments.
+    **Default:** `false`.
 * `function` {Function} The JavaScript function to call when the SQLite
-  function is invoked.
+  function is invoked. The return value of this function should be a valid
+  SQLite data type: see [Type conversion between JavaScript and SQLite][].
+  The result defaults to `NULL` if the return value is `undefined`.
 
 This method is used to create SQLite user-defined functions. This method is a
 wrapper around [`sqlite3_create_function_v2()`][].
@@ -579,10 +582,12 @@ resolution handler passed to [`database.applyChangeset()`][]. See also
 [Constants Passed To The Conflict Handler]: https://www.sqlite.org/session/c_changeset_conflict.html
 [Constants Returned From The Conflict Handler]: https://www.sqlite.org/session/c_changeset_abort.html
 [SQL injection]: https://en.wikipedia.org/wiki/SQL_injection
+[Type conversion between JavaScript and SQLite]: #type-conversion-between-javascript-and-sqlite
 [`ATTACH DATABASE`]: https://www.sqlite.org/lang_attach.html
 [`PRAGMA foreign_keys`]: https://www.sqlite.org/pragma.html#pragma_foreign_keys
 [`SQLITE_DETERMINISTIC`]: https://www.sqlite.org/c3ref/c_deterministic.html
 [`SQLITE_DIRECTONLY`]: https://www.sqlite.org/c3ref/c_deterministic.html
+[`SQLITE_MAX_FUNCTION_ARG`]: https://www.sqlite.org/limits.html#max_function_arg
 [`database.applyChangeset()`]: #databaseapplychangesetchangeset-options
 [`sqlite3_changes64()`]: https://www.sqlite.org/c3ref/changes.html
 [`sqlite3_close_v2()`]: https://www.sqlite.org/c3ref/close.html