`contributes.languages.filenamePatterns` is not documented

[brettcannon]

I had to go digging into https://github.com/microsoft/vscode/blob/7c1c5300513617d79e1c66c8d2117a5b95259ffa/src/vs/workbench/services/language/common/languageService.ts#L67-L72 to check what is exactly accepted for the setting (and that's only because I have used the setting before, else I wouldn't know about it from the docs).