SAM: Document session creation responses for subsessions

[aargh]

In the "SAM Sessions / Session Creation Response" section of docs/api/samv3

In SAM 3.3 or higher, after a style=MASTER session has been created and a subsequent subsession "SESSION ADD" message is sent, I've noticed that…

*ID=$nickname appears to be optional, because it is sent upon success when a subsession is created, but not when a new master session is created
DESTINATION=$privkey appears to now be optional instead of required because it is omitted when a subsession is created and only sent when a master session is added
*MESSAGE=$message appears to be optional because it can be sent when a subsession creation was successful (e.g.: MESSAGE=ADD $nickname)

*denotes parameters that appear to be omitted from that particular section of the doc

Alternatively, maybe rename the "Creating a Subsession" section to "Subsession Creation Request" and add a new "Subsession Creation Response" section afterwards with the parameters for better clarity.

[zzz]

also the possibility these are bugs in the code, not the doc…

[idk]

I will be comparing and contrasting messages given by I2P and i2pd's SAM implementations this week.