Fixed the help text for raw commands like "expr"

to include -- in sample command lines.  Now LLDB
prints

  expression [-f <format>] -- <expr>

instead of

  expression [-f <format>] <expr>

and also adds a new example line:

  expression <expr>

to show that in the absense of arguments the --
can be ommitted.

llvm-svn: 147540