[R-package] cleaned up formatting in R C/C++ files

[jameslamb]

This is a style-only PR. As I've been working on #629 , I see a few opportunities to make the current R-specific parts of lib_lightgbm a bit easier to read and edit. The diff looks a bit weird here on GitHub, but to be clear I did not change any of what the code in these files does. In this PR, I'm just proposing some indentation changes, for example taking definitions like this:

LIGHTGBM_C_EXPORT LGBM_SE LGBM_DatasetCreateFromFile_R(LGBM_SE filename,
  LGBM_SE parameters,
  LGBM_SE reference,
  LGBM_SE out,
  LGBM_SE call_state);

and turning them into this

LIGHTGBM_C_EXPORT LGBM_SE LGBM_DatasetCreateFromFile_R(
  LGBM_SE filename,
  LGBM_SE parameters,
  LGBM_SE reference,
  LGBM_SE out,
  LGBM_SE call_state
);

I also changed the language in the comment at the top of R_object_helper.h to better reflect the current state of the R package.

[jameslamb]

@guolinke @Laurae2 what do you think? Since I'm touching a lot of code for the R package, I don't want to merge without one of you approving.

[jameslamb]

Thanks for the reviews!

Made the requested minor change in f2c0f8f

[github-actions]

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