♻️ rework some help strings (#12306)

# Description

I changed some help outputs:


![image](https://github.com/nushell/nushell/assets/30557287/d25f0fbe-ffc3-43b3-93cf-b1793d2351b6)


![image](https://github.com/nushell/nushell/assets/30557287/acb5581c-8959-4916-8a55-d3ed6a2e1dcf)

I also checked with `rg` the commands deprecated in #9840 and found a
help or a comment here and there.

# User-Facing Changes

same of above

# Tests + Formatting

crates/nu-command/tests/main.rs

@@ -233,7 +233,7 @@ fn usage_start_uppercase() {
 
         // Check lowercase to allow usage to contain syntax like:
         //
-        // "`let-env FOO = ...` …"
+        // "`$env.FOO = ...`"
         if usage.starts_with(|u: char| u.is_lowercase()) {
             failures.push(format!("{cmd_name}: \"{usage}\""));
         }