Formerly parental-agents grammar was an exception and it did not
auto-generate itself from source code. From now on it is generated using
the same mechanism as other grammars.
For consistency with rest of the system, I've also renamed the grammar
file and the link anchors from "parentals" to "parental-agents".
Technically this is fixup for commit
0311705d4b.
Related: !5234
(cherry picked from commit 34a3b35b08)