binary-cache-store: Update compression setting documentation to match reality

The docs were out of date. Since 8a0c00b856 Nix
supports all compression algorithms exposed by libarchive (if it's built with
native support for them). Let's be honest about it in the docs.

src/libstore/include/nix/store/binary-cache-store.hh

@@ -19,7 +19,13 @@ struct BinaryCacheStoreConfig : virtual StoreConfig
     using StoreConfig::StoreConfig;
 
     const Setting<std::string> compression{
-        this, "xz", "compression", "NAR compression method (`xz`, `bzip2`, `gzip`, `zstd`, or `none`)."};
+        this,
+        "xz",
+        "compression",
+        R"(
+          NAR compression method. One of: `xz`, `bzip2`, `gzip`, `zstd`, `none`, `br`, `compress`, `grzip`, `lrzip`, `lz4`, `lzip`, `lzma` or `lzop`.
+          To use a particular compression method Nix has to be built with a version of libarchive that natively supports that compression algorithm.
+        )"};
 
     const Setting<bool> writeNARListing{
         this, false, "write-nar-listing", "Whether to write a JSON file that lists the files in each NAR."};