forgot to document GLUT_ACTION_GLUTMAINLOOP_RETURNS when discussing glutLeaveMainLoop in api docs

git-svn-id: https://svn.code.sf.net/p/freeglut/code/trunk@1715 7f0cb862-5218-0410-a997-914c9d46530a
Merge remote-tracking branch 'svn/trunk' into git_master