suggest using Plots.__init__() (#750)

This solves issue in https://discourse.julialang.org/t/plots-do-not-display-unless-i-call-gui-when-using-custom-sysimage/92207

Co-authored-by: Dilum Aluthge <dilum@aluthge.com>

Author: rdiaz02

docs/src/examples/plots.md

@@ -49,7 +49,11 @@ julia> @time display(p);
   0.331869 seconds (278.38 k allocations: 7.900 MiB)
 ```
 
-which is a sizeable speedup.
+which is a sizeable speedup. 
+
+When using the sysimage, you may need to call `Plots.__init__()` after loading
+Plots for the plots to display; otherwise, you might need to use `gui` or 
+pass argument `show = true` to `plot`.
 
 Note that since we have more stuff in our sysimage, Julia is slightly slower to
 start (0.35 seconds on this machine):