clarify that time_ns is monotonic (#57129)

Author: stevengj

base/Base_compiler.jl

@@ -188,8 +188,12 @@ end
 """
     time_ns()::UInt64
 
-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.
+Get the time in nanoseconds relative to some machine-specific arbitrary time in the past.
+The primary use is for measuring elapsed times during program execution. The return value is guaranteed to
+be monotonic (mod 2⁶⁴) while the system is running, and is unaffected by clock drift or changes to local calendar time,
+but it may change arbitrarily across system reboots or suspensions.
+
+(Although the returned time is always in nanoseconds, the timing resolution is platform-dependent.)
 """
 time_ns() = ccall(:jl_hrtime, UInt64, ())