Document COMPOSER_DISABLE_XDEBUG_WARN variable

doc/articles/troubleshooting.md

@@ -153,8 +153,10 @@ This issue can also happen on cPanel instances, when the shell fork bomb protect
 
 To improve performance when the xdebug extension is enabled, Composer automatically restarts PHP without it.
 You can override this behavior by using an environment variable: `COMPOSER_ALLOW_XDEBUG=1`.
-Composer will always show a warning if xdebug is being used. If you see this unexpectedly, then
-the restart process has failed: please report this [issue](https://github.com/composer/composer/issues).
+
+Composer will always show a warning if xdebug is being used, but you can override this with an environment variable:
+`COMPOSER_DISABLE_XDEBUG_WARN=1`. If you see this warning unexpectedly, then the restart process has failed:
+please report this [issue](https://github.com/composer/composer/issues).
 
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