[Sync EN] Update `ReturnTypeWillChange` intro for clarity (#5614)

[github-actions]

Commit: https://github.com/php/doc-en/commit/52e88759354dd18104d8c067780be9f582f20136
PR: https://github.com/php/doc-en/pull/5614

Archivos ES a actualizar

• language/predefined/attributes/returntypewillchange.xml
• reference/reflection/reflectionfunctionabstract/gettentativereturntype.xml
• reference/reflection/reflectionfunctionabstract/hastentativereturntype.xml