[docs] Added pure flag docs (#1411)

## Summary
Updated docs to add `--pure` to it. It slipped my mind to update it when
we added the feature. I was reminded by [this
comment](#1029 (comment))

## How was it tested?
N/A

Author: mohsenari

docs/app/docs/cli_reference/devbox_global_shellenv.md

@@ -19,6 +19,7 @@ Print shell commands that add global Devbox packages to your PATH
 <!-- Markdown Table of Options -->
 | Option | Description |
 | --- | --- |
+| `--pure` | If this flag is specified, devbox creates an isolated environment inheriting almost no variables from the current environment. A few variables, in particular HOME, USER and DISPLAY, are retained. |
 | `-h, --help` | help for shellenv |
 | `-q, --quiet` | suppresses logs |
 

docs/app/docs/cli_reference/devbox_shell.md

@@ -18,6 +18,7 @@ devbox shell [<dir>] [flags]
 |  `-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 |
 | `--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 |
 | `-q, --quiet` | Quiet mode: Suppresses logs. |
 

docs/app/docs/cli_reference/devbox_shellenv.md

@@ -14,6 +14,7 @@ devbox shellenv [flags]
 | `-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 |
+| `--pure` | If this flag is specified, devbox creates an isolated environment inheriting almost no variables from the current environment. A few variables, in particular HOME, USER and DISPLAY, are retained. |
 | `-h, --help` | help for shellenv |
 | `-q, --quiet` | suppresses logs |