Make it clear that we need to enable omniauth for SAML and Bitbucket

What does this MR do?

Make it clear that we need to enable omniauth for SAML and Bitbucket

Why was this MR needed?

Following the proposal mentioned in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20302#note_86116711 that we:

• Update the integration doc to make this more clear (in %11.1 or %11.0)
• Fix both integration to be disabled if omniauth.enabled sets to false, in %11.2, and reserve a spot on the release post, which I think should be enough for people to troubleshooting.

Review links:

• http://review-docs-docs-n2c9ao.178.62.207.141.xip.io/ce/integration/saml.html
• http://review-docs-docs-n2c9ao.178.62.207.141.xip.io/ce/integration/bitbucket.html

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Conform by the code review guidelines
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Documentation maintainer

What are the relevant issue numbers?

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/48677 and https://gitlab.com/gitlab-org/gitlab-ee/issues/6591