Update description text of "Merge requests" section of general project settings page

What does this MR do?

Updates the description text under the Merge request heading on the project settings page.

Current text

Choose your merge method, options, checks, and set up a default merge request description template.

New text

When the issuable_default_templates feature is available:

Choose your merge method, merge options, merge checks, and set up a default description template for merge requests.

When the issuable_default_templates is not available:

Choose your merge method, merge options, and merge checks.

Closes #11098 (closed)

This MR has a CE backport

gitlab-org/gitlab-ce!27838