docs(lsp): update `LanguageId` docs to target the file extension first (#26068)

Updated the `LanguageId` docs, to reflect how the real implementation works.
We did a "rollback" to check for the file extension first, to align with the CLI.
This was specially a problem for jsonc LanguageId with json file extension, where a trailing comma was always added by the editor.

Related https://github.com/oxc-project/oxc/issues/25893