Merge branch 'fix/ci-config-hidden-key-validation' into 'master'

Do not enforce using a hash with hidden ci key

## What does this MR do?

This MR remove the validation rule that was to strict.

## Why was this MR needed?

Enforcing hidden CI job configuration to be a hash, narrows options of using YAML anchors, and was not backwards compatbile.

## 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)
- Tests
  - [x] Added for this feature/bug
  - [ ] All builds are passing
- [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)

## What are the relevant issue numbers?

Closes #21512

See merge request !6079

1 files changed, 12 insertions, 9 deletions

diff --git a/doc/ci/yaml/README.md b/doc/ci/yaml/README.md
index 01d71088543..35eb30ed625 100644
--- a/doc/ci/yaml/README.md
+++ b/doc/ci/yaml/README.md
@@ -39,7 +39,7 @@ If you want a quick introduction to GitLab CI, follow our
     - [before_script and after_script](#before_script-and-after_script)
 - [Git Strategy](#git-strategy)
 - [Shallow cloning](#shallow-cloning)
-- [Hidden jobs](#hidden-jobs)
+- [Hidden keys](#hidden-keys)
 - [Special YAML features](#special-yaml-features)
     - [Anchors](#anchors)
 - [Validate the .gitlab-ci.yml](#validate-the-gitlab-ci-yml)
@@ -934,24 +934,27 @@ variables:
   GIT_DEPTH: "3"
 ```
 
-## Hidden jobs
+## Hidden keys
 
 >**Note:**
 Introduced in GitLab 8.6 and GitLab Runner v1.1.1.
 
-Jobs that start with a dot (`.`) will be not processed by GitLab CI. You can
+Keys that start with a dot (`.`) will be not processed by GitLab CI. You can
 use this feature to ignore jobs, or use the
-[special YAML features](#special-yaml-features) and transform the hidden jobs
+[special YAML features](#special-yaml-features) and transform the hidden keys
 into templates.
 
-In the following example, `.job_name` will be ignored:
+In the following example, `.key_name` will be ignored:
 
 ```yaml
-.job_name:
+.key_name:
   script:
     - rake spec
 ```
 
+Hidden keys can be hashes like normal CI jobs, but you are also allowed to use
+different types of structures to leverage special YAML features.
+
 ## Special YAML features
 
 It's possible to use special YAML features like anchors (`&`), aliases (`*`)
@@ -967,7 +970,7 @@ Introduced in GitLab 8.6 and GitLab Runner v1.1.1.
 
 YAML also has a handy feature called 'anchors', which let you easily duplicate
 content across your document. Anchors can be used to duplicate/inherit
-properties, and is a perfect example to be used with [hidden jobs](#hidden-jobs)
+properties, and is a perfect example to be used with [hidden keys](#hidden-keys)
 to provide templates for your jobs.
 
 The following example uses anchors and map merging. It will create two jobs,
@@ -975,7 +978,7 @@ The following example uses anchors and map merging. It will create two jobs,
 having their own custom `script` defined:
 
 ```yaml
-.job_template: &job_definition  # Hidden job that defines an anchor named 'job_definition'
+.job_template: &job_definition  # Hidden key that defines an anchor named 'job_definition'
   image: ruby:2.1
   services:
     - postgres
@@ -1081,7 +1084,7 @@ test:mysql:
     - ruby
 ```
 
-You can see that the hidden jobs are conveniently used as templates.
+You can see that the hidden keys are conveniently used as templates.
 
 ## Validate the .gitlab-ci.yml