[Sync EN] Fix GH-1132: Document mb_strlen invalid encoding behavior change in P…

PR: `https://github.com/php/doc-en/pull/5418` (2026-08-27)

**Файлы RU для обновления**
- `language-snippets.ent`
- `reference/mbstring/functions/mb-encoding-aliases.xml`
- `reference/mbstring/functions/mb-internal-encoding.xml`
- `reference/mbstring/functions/mb-strlen.xml`

