Add missing cmd to nu command (#1634)

Add if file exists syntax

Author: Kissaki

book/coming_from_cmd.md

@@ -31,7 +31,7 @@ This table was last updated for Nu 0.67.0.
 | `GOTO`                               |                                                                                     | Jump to a label                                                       |
 | `IF ERRORLEVEL <number> <command>`   | `if $env.LAST_EXIT_CODE >= <number> { <command> }`                                  | Run a command if the last command returned an error code >= specified |
 | `IF <string> EQU <string> <command>` | `if <string> == <string> { <command> }`                                             | Run a command if strings match                                        |
-| `IF EXIST <filename> <command>`      |                                                                                     | Run a command if the file exists                                      |
+| `IF EXIST <filename> <command>`      | `if (<filename> \| path exists) { <command> }`                                      | Run a command if the file exists                                      |
 | `IF DEFINED <variable> <command>`    |                                                                                     | Run a command if the variable is defined                              |
 | `MD` or `MKDIR`                      | `mkdir`                                                                             | Create directories                                                    |
 | `MKLINK`                             |                                                                                     | Create symbolic links                                                 |