patch 9.0.1411: accuracy of profiling is not optimal

Problem:    Accuracy of profiling is not optimal.
Solution:   Use CLOCK_MONOTONIC if possible. (Ernie Rael, closes #12129)
diff --git a/runtime/doc/builtin.txt b/runtime/doc/builtin.txt
index 0bdeabf..ec680be 100644
--- a/runtime/doc/builtin.txt
+++ b/runtime/doc/builtin.txt
@@ -7150,7 +7150,8 @@
 			call MyFunction()
 			echo reltimestr(reltime(start))
 <		Note that overhead for the commands will be added to the time.
-		The accuracy depends on the system.
+		The accuracy depends on the system. Use reltimefloat() for the
+		greatest accuracy which is nanoseconds on some systems.
 		Leading spaces are used to make the string align nicely.  You
 		can use split() to remove it. >
 			echo split(reltimestr(reltime(start)))[0]
@@ -10753,6 +10754,7 @@
 postscript		Compiled with PostScript file printing.
 printer			Compiled with |:hardcopy| support.
 profile			Compiled with |:profile| support.
+prof_nsec		Profile results are in nano seconds.
 python			Python 2.x interface available. |has-python|
 python_compiled		Compiled with Python 2.x interface. |has-python|
 python_dynamic		Python 2.x interface is dynamically loaded. |has-python|