[zircon][docs] Improve description of clock_get_monotonic and uptime

Added verbiage describing the limitations of depending on
clock_get_monotonic as a source of uptime.

Test: No behavior change.

Change-Id: If40a5ad09fcd333a9b4c8020c0f92558cc0f181b

zircon/docs/syscalls/clock_get_monotonic.md

@@ -20,7 +20,8 @@ zx_time_t zx_clock_get_monotonic(void);
 
 `zx_clock_get_monotonic()` returns the current time in the system
 monotonic clock. This is the number of nanoseconds since the system was
-powered on.
+powered on. It does not always reset on reboot and does not adjust during
+sleep, and thus should not be used as a reliable source of uptime.
 
 ## RIGHTS