Added basic docs for the Emoji Rake tasks

## What does this MR do?

This MR adds basic documentation for the two Emoji Rake tasks.

## Are there points in the code the reviewer needs to double check?

Spelling, that sort of thing.

## Why was this MR needed?

These Rake tasks only had some small amount of source level documentation.

## What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/14794

## Does this MR meet the acceptance criteria?

- [x] ~~[CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added~~
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !4877