Update documentation of the qt_add_translations command

Suggestion to update the Plural Forms section of the qt_add_translations documentation (https://doc.qt.io/qt-6/qtlinguist-cmake-qt-add-translations.html#plural-forms) in order to provide a clearer explanation on the generation of the plurals-only ts file for the source language and the purpose and use of the PLURALS_TS_FILE argument. As the current wording of the section could lead to confusion for the user.

For instance, the following statement could cause confusion as it makes no mention that this is specifically for the source language: "For handling plural forms correctly, create an additional .ts file for that language that only contains translatable strings for plural forms".

Also, the lack of example on the usage of the PLURALS_TS_FILE argument makes it difficult to understand its purpose. Specially, when the qt_add_translations command automatically generates a plurals-only .ts file for the source language when translatable strings which contain plurals exists in the source target.