help generate_docs | flatten crashes nushell (#3099)

* fix case where parent_name was {nu, term} and possibly others in the future by doing an extra test first to see if if the *parent_name key actually exists in cmap

* update with help generate_docs testing

crates/nu-command/tests/commands/mod.rs

@@ -22,6 +22,7 @@ mod get;
 mod group_by;
 mod hash_;
 mod headers;
+mod help;
 mod histogram;
 mod insert;
 mod into_int;