General/Merge requests - Review and revise settings-related UI text

What does this MR do?

TW review of user-facing text in Settings > General > Merge requests (for example, here).

Test URLs:

• Current: https://gitlab.com/gitlab-org/gitlab/edit#js-merge-request-settings
• Test: http://127.0.0.1:3001/root/gitlab-shell/edit#js-merge-request-settings

Resolve technical-writing#310

Related technical-writing#278

Screenshots (strongly suggested)

Intro (no change)

Promo banner

Before	After
	[diff] Improve merge requests and customer support with GitLab Enterprise Edition. Merge request approvals Set the number of necessary approvals and define a list of approvers needed for every merge request in a project.

Merge method

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Merge options

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Squash commits when merging

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Merge checks

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Merge suggestions

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Default description template for merge requests

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Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team