diff -u b/core/modules/system/theme.api.php b/core/modules/system/theme.api.php
--- b/core/modules/system/theme.api.php
+++ b/core/modules/system/theme.api.php
@@ -847,9 +847,9 @@
 }
 
 /**
- * Alter libraries provided by extension.
+ * Alter libraries provided by an extension.
  *
- * Allows modules and themes to change libraries' definitions, mostly used to
+ * Allows modules and themes to change libraries' definitions; mostly used to
  * update a library to a newer version, while ensuring backward compatibility.
  * In general, such manipulations should only be done to extend the library's
  * functionality in a backward-compatible way, to avoid breaking other modules