remove duplicated docs in dapply

## What changes were proposed in this pull request?
Removed unnecessary duplicated documentation in dapply and dapplyCollect.

In this pull request I created separate R docs for dapply and dapplyCollect - kept dapply's documentation separate from dapplyCollect's and referred from one to another via a link.

## How was this patch tested?
Existing test cases.

Author: Narine Kokhlikyan <narine@slice.com>

Closes #13790 from NarineK/dapply-docs-fix.