How is "information relating to the safety of flight" defined by the FAA?
The language is intentionally vague.
Much like Justice Stewart said about pornography, when it comes to "information relating to the safety of flight" you'll know it when you see it. 


Birds flying around in the approach path? Yup.  
Wind shear on short final? Yup.
(Even if it's forecast/reported telling them how much airspeed you gained or lost helps.)
"Drone" buzzing around in the traffic pattern? Yup.
Deer grazing in the middle of a grass runway? Yup.
Bird strike? Absolutely.
(There are regs/procedures specifically for this.)
The guy in row 3J is threatening the flight attendants? Yup.
The guy in row 3J spilled his soda on his lap? …Not so much.