[sysrst_ctrl] Add more detail about the ULP wakeup feature
[sysrst_ctrl] Add more detail about the ULP wakeup feature

Resolves #10099

Signed-off-by: Michael Schaffner <a005a2d3342a687ea2d05360c953107acbe83e08@opentitan.org>

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