@kindex read-only @r{(text property)}
If a character has the property @code{read-only}, then modifying that
character is not allowed. Any command that would do so gets an error,
-@code{text-read-only}.
+@code{text-read-only}. If the property value is a string, that string
+is used as the error message.
Insertion next to a read-only character is an error if inserting
ordinary text there would inherit the @code{read-only} property due to