client: Document CacheRef syntax in help

Ref: #25

client/src/command/cache.rs

@@ -33,6 +33,9 @@ enum Command {
 #[derive(Debug, Clone, Parser)]
 struct Create {
     /// Name of the cache to create.
+    ///
+    /// This can be either `servername:cachename` or `cachename`
+    /// when using the default server.
     cache: CacheRef,
 
     /// Make the cache public.

client/src/command/push.rs

@@ -16,6 +16,9 @@ use attic::nix_store::NixStore;
 #[derive(Debug, Parser)]
 pub struct Push {
     /// The cache to push to.
+    ///
+    /// This can be either `servername:cachename` or `cachename`
+    /// when using the default server.
     cache: CacheRef,
 
     /// The store paths to push.

client/src/command/use.rs

@@ -13,6 +13,9 @@ use crate::nix_netrc::NixNetrc;
 #[derive(Debug, Parser)]
 pub struct Use {
     /// The cache to configure.
+    ///
+    /// This can be either `servername:cachename` or `cachename`
+    /// when using the default server.
     cache: CacheRef,
 }
 

client/src/command/watch_store.rs

@@ -17,6 +17,9 @@ use attic::nix_store::{NixStore, StorePath};
 #[derive(Debug, Parser)]
 pub struct WatchStore {
     /// The cache to push to.
+    ///
+    /// This can be either `servername:cachename` or `cachename`
+    /// when using the default server.
     cache: CacheRef,
 
     /// Push the new paths only and do not compute closures.