Fix a potentially serious bug discovered by BEAM.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13666 a5019735-40e9-0310-863c-91ae7b9d1cf9
Two small wording fixes in manual


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13667 a5019735-40e9-0310-863c-91ae7b9d1cf9