Document breaking change in strspn/strcspn

See https://phabricator.wikimedia.org/T264502.

[ci skip]

Author: thiemowmde

UPGRADING

@@ -532,6 +532,8 @@ PHP 8.0 UPGRADE NOTES
   . parse_str() can no longer be used without specifying a result array.
   . fgetss() has been removed.
   . The string.strip_tags filter has been removed.
+  . strspn() and strcspn() now throw a ValueError when the start or length
+    argument exceed the bounds of the string.
   . The needle argument of strpos(), strrpos(), stripos(), strripos(), strstr(),
     strchr(), strrchr(), and stristr() will now always be interpreted as a
     string. Previously non-string needles were interpreted as an ASCII code