thank you for your comments. I agree the documentation of TRandom constructor is missing in the reference doc.
It should be stressed however that users should not instantiates instances of TRandom, since the generator has bad random quality and the sequences repeats itself after only generating 10^9 numbers (which can be done in just few seconds).
Also the initialization of TRandom is different when calling with a 0 value than TRandom3. TRandom3 using a TUUID which is a unique number, while this not the case for TRandom. The seed is generate just simply using the current time.
I will update in the next days the doc taking into account your suggestions,