chore(fish): Add FIXME

About inabilities of handling nested subcommands more than 3 levels,
which I think not practical in public. Take rustup for example,
`rustup toolchain install <version>`, there is only 3 levels of nested
subcommands. But there is also the case of `rustup help toolchain install`,
or `rustup toolchain help install`.

clap_complete/src/shells/fish.rs

@@ -121,9 +121,11 @@ fn gen_fish_inner(
             [command, subcommand] => out.push_str(&format!(
                 " {command}; and __fish_seen_subcommand_from {subcommand}"
             )),
+            // FIXME: Create an `__fish_{name}_help_using_command` to check for help of subcommands of subcommand?
             [command, "help", _subcommand] => {
                 out.push_str(&format!(" {command}; and __fish_seen_subcommand_from help"));
             }
+            // FIXME: Create an `__fish_help_{name}_using_command` to check for help of `{name}` subcommand?
             ["help", command, _subcommand] => {
                 out.push_str(&format!(" help; and __fish_seen_subcommand_from {command}"));
             }