Add "API" to all API docs

doc/api/access_requests.md

-# Group and project access requests
+# Group and project access requests API
  >**Note:** This feature was introduced in GitLab 8.11

doc/api/award_emoji.md

-# Award Emoji
+# Award Emoji API
 > [Introduced][ce-4575] in GitLab 8.9, Snippet support in 8.12

doc/api/boards.md

-# Boards
+# Issue Boards API
 Every API call to boards must be authenticated.

doc/api/branches.md

-# Branches
+# Branches API
 ## List repository branches

doc/api/broadcast_messages.md

-# Broadcast Messages
+# Broadcast Messages API
 > **Note:** This feature was introduced in GitLab 8.12.

doc/api/build_variables.md

-# Build Variables
+# Build Variables  API
 ## List project variables

doc/api/ci/lint.md

-# Validate the .gitlab-ci.yml
+# Validate the .gitlab-ci.yml (API)
 > [Introduced][ce-5953] in GitLab 8.12.

doc/api/ci/runners.md

-# Runners API
+# Register and Delete Runners API
 API used by Runners to register and delete themselves.

doc/api/deploy_key_multiple_projects.md

-# Adding deploy keys to multiple projects
+# Adding deploy keys to multiple projects via API
 If you want to easily add the same deploy key to multiple projects in the same
 group, this can be achieved quite easily with the API.

doc/api/deploy_keys.md

-# Deploy Keys
+# Deploy Keys API
 ## List all deploy keys

doc/api/enviroments.md

-# Environments
+# Environments API
 ## List environments

doc/api/groups.md

-# Groups
+# Groups API
 ## List groups

doc/api/issues.md

-# Issues
+# Issues API
 Every API call to issues must be authenticated.

doc/api/keys.md

-# Keys
+# Keys API
 ## Get SSH key with user by ID of an SSH key

doc/api/labels.md

-# Labels
+# Labels API
 ## List labels

doc/api/members.md

-# Group and project members
+# Group and project members API
 **Valid access levels**

doc/api/merge_requests.md

-# Merge requests
+# Merge requests API
 ## List merge requests

doc/api/milestones.md

-# Milestones
+# Milestones API
 ## List project milestones

doc/api/namespaces.md

-# Namespaces
+# Namespaces API
 Usernames and groupnames fall under a special category called namespaces.

doc/api/notes.md

-# Notes
+# Notes API
 Notes are comments on snippets, issues or merge requests.