add small note: closing menu before executing callback is also important so that the callback can actually modify the menu

git-svn-id: https://svn.code.sf.net/p/freeglut/code/trunk@1636 7f0cb862-5218-0410-a997-914c9d46530a
timer demo now has a menu to:
- set flicker tempo
- test whether menus can be modified in the callback (there was a regression with this in freeglut 2.8.1 that was perchance already fixed in r1583, but make sure we're testing this from now on)

git-svn-id: https://svn.code.sf.net/p/freeglut/code/trunk@1637 7f0cb862-5218-0410-a997-914c9d46530a
Merge branch 'svn/trunk'