docs: avoid lint flag cardinality wording

[Teakowa]

Follow-up to #229 and PR #230.

• Rewrites “both are repeatable” as “are repeatable” so the paragraph has no mutable flag-count synchronization obligation.
• Preserves both named CLI flags, stable rule IDs, registry ownership, and all CLI/protocol/example contracts.
• Validation: git diff --check, cargo fmt --all -- --check, and cargo test -p wright-analyzer --all-targets passed.

[Teakowa]

Re-review — cleared

The remaining #229 acceptance gap is resolved. The following lint-only flags configure the registry and are repeatable: preserves the behavioral contract while removing the hidden two-item cardinality dependency introduced by both.

The one-line diff does not change flags, rule IDs, registry ownership, CLI semantics, protocol values, or illustrative payloads. The recorded diff/fmt/analyzer-test validation is sufficient for this focused follow-up.

No further findings. Ready to merge after the organization policy PR is settled.