update comments

Author: chelsea-lin

bigframes/core/compile/sqlglot/expressions/constants.py

@@ -17,6 +17,7 @@
 _ZERO = sge.Cast(this=sge.convert(0), to="INT64")
 _NAN = sge.Cast(this=sge.convert("NaN"), to="FLOAT64")
 _INF = sge.Cast(this=sge.convert("Infinity"), to="FLOAT64")
+_NEG_INF = sge.Cast(this=sge.convert("-Infinity"), to="FLOAT64")
 
 # Approx Highest number you can pass in to EXP function and get a valid FLOAT64 result
 # FLOAT64 has 11 exponent bits, so max values is about 2**(2**10)