[Sync EN] sscanf/fscanf: Clarify negative -1 return value(#5580)

Commit: `https://github.com/php/doc-en/commit/9947012f745f0fb6e083bce5a8c920213ff468b7`
PR: `https://github.com/php/doc-en/pull/5580`

**Fichiers FR à mettre à jour**
- `reference/filesystem/functions/fscanf.xml`
- `reference/spl/splfileobject/fscanf.xml`
- `reference/strings/functions/sscanf.xml`
