Add some comments why some options are hidden.

Author: ehuss

src/bin/cargo/commands/login.rs

@@ -10,6 +10,7 @@ pub fn cli() -> App {
         )
         .arg(opt("quiet", "No output printed to stdout").short("q"))
         .arg(Arg::with_name("token"))
+        // --host is deprecated (use --registry instead)
         .arg(
             opt("host", "Host to set the token for")
                 .value_name("HOST")

src/bin/cargo/commands/tree.rs

@@ -19,7 +19,9 @@ pub fn cli() -> App {
             "Display the tree for all packages in the workspace",
             "Exclude specific workspace members",
         )
+        // Deprecated, use --no-dedupe instead.
         .arg(Arg::with_name("all").long("all").short("a").hidden(true))
+        // Deprecated, use --target=all instead.
         .arg(
             Arg::with_name("all-targets")
                 .long("all-targets")
@@ -30,6 +32,7 @@ pub fn cli() -> App {
             "Filter dependencies matching the given target-triple (default host platform). \
             Pass `all` to include all targets.",
         )
+        // Deprecated, use -e=no-dev instead.
         .arg(
             Arg::with_name("no-dev-dependencies")
                 .long("no-dev-dependencies")
@@ -52,7 +55,9 @@ pub fn cli() -> App {
             )
             .short("i"),
         )
+        // Deprecated, use --prefix=none instead.
         .arg(Arg::with_name("no-indent").long("no-indent").hidden(true))
+        // Deprecated, use --prefix=depth instead.
         .arg(
             Arg::with_name("prefix-depth")
                 .long("prefix-depth")

src/bin/cargo/commands/vendor.rs

@@ -32,21 +32,25 @@ pub fn cli() -> App {
                 .long("versioned-dirs")
                 .help("Always include version in subdir name"),
         )
+        // Not supported.
         .arg(
             Arg::with_name("no-merge-sources")
                 .long("no-merge-sources")
                 .hidden(true),
         )
+        // Not supported.
         .arg(
             Arg::with_name("relative-path")
                 .long("relative-path")
                 .hidden(true),
         )
+        // Not supported.
         .arg(
             Arg::with_name("only-git-deps")
                 .long("only-git-deps")
                 .hidden(true),
         )
+        // Not supported.
         .arg(
             Arg::with_name("disallow-duplicates")
                 .long("disallow-duplicates")