improve: L-01: Comment on setCrossDomainAdmin Edge Case

[bmzig]

The ForwarderBase contract has the setCrossDomainAdmin function which changes the crossDomainAdmin state variable. This variable is used to give access control to almost all functions within the contract, including those that are meant to be used to complete L1->L3 message passing and asset transfers. When calling setCrossDomainAdmin, it is assumed that there are no outstanding operations that need to be passed to another chain. This is because when operations arrive at L2, the original sender of those is the old crossDomainAdmin and will be blocked from continuing further. In order to raise awareness about such behavior, consider documenting this edge case in the setCrossDomainAdmin function.

This PR adds a comment outlining this effect.

[james-a-morris]

nit