man: describe capability checks on the bus

man: describe capability checks on the bus

A description of SD_BUS_VTABLE_CAPABILITY is added, and the discussion
on SD_BUS_VTABLE_UNPRIVILEGED in expanded. I think it would be nice
to add longer description of how access is checked (maybe in sd-bus(3)),
but I'm leaving that for later. I think the text that was added here
describes everything, even if tersely.

Fixes #21882.

(cherry picked from commit b4e7d7555e6266ff566a17eb5f616b365771028f)