Add php-only to docs

doc/06-config.md

@@ -316,7 +316,8 @@ file.
 
 ## platform-check
 
-Defaults to `true`. If set to `false`, Composer will not create and require a
-`platform_check.php` file as part of the autoloader bootstrap.
+Defaults to `php-only` which only checks the PHP version. Set to `true` to also
+check the presence of extension. If set to `false`, Composer will not create and
+require a `platform_check.php` file as part of the autoloader bootstrap.
 
 ← [Repositories](05-repositories.md)  |  [Runtime](07-runtime.md) →

doc/07-runtime.md

@@ -99,11 +99,16 @@ by the PHP process currently running. If the requirements are not met, the scrip
 prints a warning with the missing requirements and exits with code 104.
 
 To avoid an unexpected white page of death with some obscure PHP extension warning in
-production, you can run `composer check-platform-reqs --no-dev` as part of your
+production, you can run `composer check-platform-reqs` as part of your
 deployment/build and if that returns a non-0 code you should abort.
 
+The default value is `php-only` which only checks the PHP version.
+
 If you for some reason do not want to use this safety check, and would rather
 risk runtime errors when your code executes, you can disable this by setting the
 [`platform-check`](06-config.md#platform-check) config option to `false`.
 
+If you want the check to include verifying the presence of PHP extensions,
+set the config option to `true`.
+
 ← [Config](06-config.md)  |  [Community](08-community.md) →