[translations] A couple of untranslated strings in 5.x core

[Cog Rusty]

Question: While trying to understand better the problem with the
/admin/user/access page where strings like "edit your own [type]
content" appear completely untranslated for imported or user-defined
types if a translation for them has not been imported, I stumbled into
the following:

Apparently the translation is performed in user.module, line 1823:

$form['permission'][$perm] = array('#value' => t($perm));

This displays the translation of the *whole* permission string IFF it
already exists. So far so good. However the existing string "edit own
book pages", which is included in book-module.pot, does not have a t()
in book.module. How does potx find it and extract it?