[SPARK-18349][SPARKR] Update R API documentation on ml model summary

## What changes were proposed in this pull request?
In this PR, the document of `summary` method is improved in the format:

returns summary information of the fitted model, which is a list. The list includes .......

Since `summary` in R is mainly about the model, which is not the same as `summary` object on scala side, if there is one, the scala API doc is not pointed here.

In current document, some `return` have `.` and some don't have. `.` is added to missed ones.

Since spark.logit `summary` has a big refactoring, this PR doesn't include this one. It will be changed when the `spark.logit` PR is merged.

## How was this patch tested?

Manual build.

Author: wm624@hotmail.com <wm624@hotmail.com>

Closes #16150 from wangmiao1981/audit2.