Fix/improve user manual and man page derived from it.

The current manual and man page tells:
      --tool=<toolname> [default: memcheck]
           Run the Valgrind tool called toolname, e.g. Memcheck, Cachegrind, etc.

where the toolname examples do not list all the tools, and use uppercase
first letter, which is not understood.

So, use lower case, and give the list of all known tools.



git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13920 a5019735-40e9-0310-863c-91ae7b9d1cf9