Document the textDocument/inactiveRegions extension #104
Description
In https://reviews.llvm.org/D143974, clangd added support for a textDocument/inactiveRegions request which is an unofficial extension to the Language Server Protocol (with accompanying client-side support in vscode-clangd in clangd/vscode-clangd#193).
Other extensions are documented at https://clangd.llvm.org/extensions; we should document inactiveRegions there as well.