So far the units there were being documented had only one custom interface.
But for the pid1 case, something more flexibile is needed. So let's add
an annotation in the page what we want to print, and filter in the generator.
As usual, the formatting was fixed and various obvious updates
were done, but nothing major.
I removed documentation of snapshots and related methods though.
Zbigniew Jędrzejewski-Szmek