* Display aliases and custom commands in which; Fix#2810
Example output of nu after the commit is applied:
```shell
/home/leo/repos/nushell(feature/which_inspect_alias)> def docker-ps [] { docker ps --format '{{json .}}' | from json -o }
/home/leo/repos/nushell(feature/which_inspect_alias)> which docker-ps
───┬───────────┬────────────────────────┬─────────
# │ arg │ path │ builtin
───┼───────────┼────────────────────────┼─────────
0 │ docker-ps │ nushell custom command │ No
───┴───────────┴────────────────────────┴─────────
/home/leo/repos/nushell(feature/which_inspect_alias)> alias d = gid pd
/home/leo/repos/nushell(feature/which_inspect_alias)> which d
───┬─────┬───────────────┬─────────
# │ arg │ path │ builtin
───┼─────┼───────────────┼─────────
0 │ d │ nushell alias │ No
───┴─────┴───────────────┴─────────
```
* Update documentation
48f535f02e