Clarify that OmniAuth needs enabling only for < 11.4

doc/integration/bitbucket.md

 # Integrate your GitLab server with Bitbucket Cloud
 NOTE: **Note:**
-You need to [enable OmniAuth](omniauth.md) in order to use this.
+Starting from GitLab 11.4, OmniAuth is enabled by default. If you're using an
+earlier version, you'll need to explicitly enable it.
 Import projects from Bitbucket.org and login to your GitLab instance with your
 Bitbucket.org account.

doc/integration/saml.md

 # SAML OmniAuth Provider
-> This topic is for SAML on self-managed GitLab instances. For SAML on GitLab.com, see [SAML SSO for GitLab.com Groups](../user/group/saml_sso/index.md).
-NOTE: **Note:**
-You need to [enable OmniAuth](omniauth.md) in order to use this.
+Note that:
+- SAML OmniAuth Provider is for SAML on self-managed GitLab instances. For SAML on
+  GitLab.com, see [SAML SSO for GitLab.com Groups](../user/group/saml_sso/index.md).
+- Starting from GitLab 11.4, OmniAuth is enabled by default. If you're using an
+  earlier version, you'll need to explicitly enable it.
 GitLab can be configured to act as a SAML 2.0 Service Provider (SP). This allows
 GitLab to consume assertions from a SAML 2.0 Identity Provider (IdP) such as

doc/integration/shibboleth.md

@@ -16,7 +16,7 @@ The following changes are needed to enable Shibboleth:
 1. Protect OmniAuth Shibboleth callback URL:
-   ```
+   ```apache
    <Location /users/auth/shibboleth/callback>
      AuthType shibboleth
      ShibRequestSetting requireSession 1
@@ -36,7 +36,7 @@ The following changes are needed to enable Shibboleth:
 1. Exclude Shibboleth URLs from rewriting. Add `RewriteCond %{REQUEST_URI} !/Shibboleth.sso` and `RewriteCond %{REQUEST_URI} !/shibboleth-sp`. Config should look like this:
-   ```
+   ```apache
    # Apache equivalent of Nginx try files
    RewriteEngine on
    RewriteCond %{DOCUMENT_ROOT}/%{REQUEST_FILENAME} !-f
@@ -46,8 +46,12 @@ The following changes are needed to enable Shibboleth:
    RequestHeader set X_FORWARDED_PROTO 'https'
    ```
-1. Edit `/etc/gitlab/gitlab.rb` configuration file to enable OmniAuth and add
-   Shibboleth as an OmniAuth provider. User attributes will be sent from the
+   **NOTE:**
+   Starting from GitLab 11.4, OmniAuth is enabled by default. If you're using an
+   earlier version, you'll need to explicitly enable it in `/etc/gitlab/gitlab.rb`.
+
+1. In addition, add Shibboleth to `/etc/gitlab/gitlab.rb` as an OmniAuth provider.
+   User attributes will be sent from the
    Apache reverse proxy to GitLab as headers with the names from the Shibboleth
    attribute mapping. Therefore the values of the `args` hash
    should be in the form of `"HTTP_ATTRIBUTE"`. The keys in the hash are arguments