clarify in what situations "however" is allowed according to Docs writing guidelines

**Issue description:** "however" is one of the 8 banned words according to [the writing guidelines](https://docs.godotengine.org/en/stable/community/contributing/docs_writing_guidelines.html#ban-these-8-words). _It also says that it has "some uses", but it's never explained in what case it may be used._

**URL to the documentation page (if already existing):** https://docs.godotengine.org/en/stable/community/contributing/docs_writing_guidelines.html#ban-these-8-words