Updated devbox shell documentation (#2074)

## Summary
- added --config flag;
- added --environment flag;
- removed dirs path param;

## How was it tested?
Checked against the command `devbox shell -h` with the latest version.

Signed-off-by: Valerio Farrotti <[email protected]>

Author: vallops99

docs/app/docs/cli_reference/devbox_shell.md

@@ -4,19 +4,23 @@ Start a new shell or run a command with access to your packages
 
 ## Synopsis
 
-Start a new shell or run a command with access to your packages. The interactive shell will use the devbox.json in your current directory, or the directory provided with `dir`.
+Start a new shell or run a command with access to your packages.   
+If the --config flag is set, the shell will be started using the devbox.json found in the --config flag directory.   
+If --config isn't set, then devbox recursively searches the current directory and its parents.
 
 ```bash
-devbox shell [<dir>] [flags]
+devbox shell [flags]
 ```
 
 ## Options
 
 <!-- Markdown Table of Options -->
 | Option | Description |
 | --- | --- |
+|  `-c, --config string`|  path to directory containing a devbox.json config file |
 |  `-e, --env stringToString` |  environment variables to set in the devbox environment (default []) |
 |  `--env-file string` | path to a file containing environment variables to set in the devbox environment |
+|  `--environment string` | environment to use, when supported (e.g.secrets support dev, prod, preview.) (default "dev") |
 | `--print-env` | Print a script to setup a devbox shell environment |
 | `--pure` | If this flag is specified, devbox creates an isolated shell inheriting almost no variables from the current environment. A few variables, in particular HOME, USER and DISPLAY, are retained. |
 | `-h, --help` | help for shell |