add docs for SKIP_PREFLIGHT_CHECK

Adds entry in docusaurus site. Provides additional context in `verifyPackageTree` error messaging.

Closes facebook#11119.

Author: eckdanny

docusaurus/docs/advanced-configuration.md

@@ -30,3 +30,4 @@ You can adjust various development and production settings by setting environmen
 | ESLINT_NO_DEV_ERRORS      |   ✅ Used   | 🚫 Ignored | When set to `true`, ESLint errors are converted to warnings during development. As a result, ESLint output will no longer appear in the error overlay.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
 | DISABLE_ESLINT_PLUGIN     |   ✅ Used   |  ✅ Used   | When set to `true`, [eslint-webpack-plugin](https://github.com/webpack-contrib/eslint-webpack-plugin) will be completely disabled.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
 | DISABLE_NEW_JSX_TRANSFORM |   ✅ Used   |  ✅ Used   | When set to `true`, disables the [new JSX transform](https://reactjs.org/blog/2020/09/22/introducing-the-new-jsx-transform.html) introduced in React 17 and backported to React 16.14.0, 15.7.0, and 0.14.10. New projects will use a version of React that supports this by default but you may need to disable it in existing projects if you can't upgrade React.                                                                                                                                                                                                                                                                                                     |
+| SKIP_PREFLIGHT_CHECK      |   ✅ Used   | 🚫 Ignored | By default, Create React App will scan your dependency tree for version mismatches likely to break your setup. When set to `false`, the scan is skipped. In non-traditional project setups (<tt>lerna</tt>, <tt>yarn workspaces</tt>), this can generate false positives.                                                                                                                                                                                                                                                                                                                                                                                                |

packages/react-scripts/scripts/utils/verifyPackageTree.js

@@ -99,7 +99,8 @@ function verifyPackageTree() {
             `  ${chalk.bold(chalk.red(maybeDep))} (version: ${chalk.bold(
               chalk.red(depPackageJson.version)
             )}) \n\n` +
-            `Manually installing incompatible versions is known to cause hard-to-debug issues.\n\n` +
+            `Manually installing incompatible versions is known to cause hard-to-debug issues.\n` +
+            `If you are using Create React App in a multi-project setup, you may want to ignore this check.\n\n` +
             chalk.red(
               `If you would prefer to ignore this check, add ${chalk.bold(
                 'SKIP_PREFLIGHT_CHECK=true'