docs: correct setError error argument type signature

[dfedoryshchev]

The setError docs describe the second argument as FieldError with type and types marked as required. Both are wrong against the library source.

The argument type is ErrorOption, not FieldError (see UseFormSetError in src/types/form.ts:644). All three fields on ErrorOption are optional (src/types/errors.ts:21-25):

export type ErrorOption = {
  message?: Message;
  type?: LiteralUnion<keyof RegisterOptions, string>;
  types?: MultipleFieldErrors;
};

FieldError is a different type (the shape of entries on formState.errors), so the current wording also pointed at the wrong thing.

Two content changes in src/content/docs/useform/seterror.mdx: the signature line (error: FieldError to error: ErrorOption) and the error props row (optional markers on type and types). Prettier reflowed the table column widths; split into a second commit matching the repo's format-as-follow-up pattern.

Addresses point 1 from #1065. The other points (types usage details, "Multiple Errors" naming, nested-merge behavior) need separate verification and are out of scope here.

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