update docstring for time_ns (#54726)

See discussion in
#54696 (comment).

Author: KristofferC

base/Base.jl

@@ -152,7 +152,8 @@ end
 """
     time_ns() -> UInt64
 
-Get the time in nanoseconds. The time corresponding to 0 is undefined, and wraps every 585 years.
+Get the time in nanoseconds relative to some arbitrary time in the past. The primary use is for measuring the elapsed time
+between two moments in time.
 """
 time_ns() = ccall(:jl_hrtime, UInt64, ())