Message about rebuilding node_access tables is confusing for end users

The message is not confusing, nor scary, as it reports exactly what it needs to be done in order to not have access problems.

Other modules report a similar message, even it can change the place where the message is put. Rather than putting the message in a troubleshooting page, I prefer to keep it in the settings page, as the user can simply click the link, and have the permissions rebuilt automatically.
Before, the module rebuilt the permissions automatically, but that was done every time the form button was clicked; that meant the permissions keep to be rebuilt every time one new user was added, which caused performing reduction.

The message is not for end users, but for users with the permission to administer the book access permissions; such permission should not be given to not trusted users, or generically to all users with access to a website.