clang-format: improve vim integration docs

Improve the documentation for vim integration of clang-format.  Prefer the use
of <c-o> to do the normal mode command execution to avoid side-effects of the
escape and re-insertion (cursor movement).  Tweak the macros to use a double
return to avoid having to manually return control to the editor from the
subprocess.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@220685 91177308-0d34-0410-b5e6-96231b3b80d8