Merge branch 'document-emoji-rake-tasks' into 'master'

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

doc/development/rake_tasks.md

@@ -33,3 +33,23 @@ bundle exec rake gitlab:generate_docs
 ```
 bundle exec rake services:doc
 ```
+
+## Updating Emoji Digests
+
+To update the Emoji digests file (used for Emoji autocomplete) you must run the
+following:
+
+```
+bundle exec rake gemojione:digests
+```
+
+This will update the file `fixtures/emojis/digests.json` based on the currently
+available Emoji.
+
+## Emoji Sprites
+
+Generating a sprite file containing all the Emoji can be done by running:
+
+```
+bundle exec rake gemojione:sprite
+```