Bootstrap/Load: Give more context and warning about editing compat.php.

As indicated by name, this is a compatibility file which warrants more care to begin with, but it's still worth warning folks about how narrow function availability is in this file.

Props jorbin, dmsnell, helen.
See #61694.


git-svn-id: https://develop.svn.wordpress.org/trunk@59043 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/compat.php

@@ -2,6 +2,11 @@
 /**
  * WordPress implementation for PHP functions either missing from older PHP versions or not included by default.
  *
+ * This file is loaded extremely early and the functions can be relied upon by drop-ins.
+ * Ergo, please ensure you do not rely on external functions when writing code for this file.
+ * Only use functions built into PHP or are defined in this file and have adequate testing
+ * and error suppression to ensure the file will run correctly and not break websites.
+ *
  * @package PHP
  * @access private
  */