[pwrmgr] Documentation update
[pwrmgr] Documentation update

companion PR to #9555 and clarifies `pwrmgr` and `rom_ctrl` interaction.

Signed-off-by: Timothy Chen <d7b2ae30d8c0f215facd721941273067fd1d14a2@google.com>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan