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: http://llvm.org/svn/llvm-project/cfe/trunk@220685 91177308-0d34-0410-b5e6-96231b3b80d8
clang-format: Don't break after very short return types.

Before:
  void
  SomeFunction(int parameter);

After:
  void SomeFunction(
      int parameter);

(Unless AlwaysBreakAfterDefinitionReturnType after type is set).

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@220686 91177308-0d34-0410-b5e6-96231b3b80d8