[llvm-mca][docs] Improve the CommandLine documentation.

This patch replaces all the remaining occurrences of string "MCA" with
":program:`llvm-mca`".  Somehow I missed those strings when I committed r338394.

This patch also improves section "Instruction Dispatch".


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@338881 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed