Fix dead links in the docs (Jun 2016)

## What does this MR do?

Fix dead links in the docs found as of Jun 2016.

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

n/a

## Why was this MR needed?

This MR must improve UX on docs.gitlab.com.

## What are the relevant issue numbers?

Closes #19156 

## Does this MR meet the acceptance criteria?

- No [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) updated
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [x] 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 !4921

Amend jira service api docs [ci skip]

## What does this MR do?

Amends the API documentation for jira service integration

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

No

## Why was this MR needed?

The current documentation omits the username and password parameters

## What are the relevant issue numbers?

## Screenshots (if relevant)

See merge request !4382

[ci skip]