Clarify changelog guidance documentation

What does this MR do?

Previously the documented guidance for "What warrants a changelog entry" mixed a variety of terminology (must/should) and structure, and placed the most important instructions in the middle of the list. This attempts to place the most important instructions at the top of the list, standardize the structure of the list, and standardize the terminology used (must/must not). The intent is to make this guidance faster and easier to take conclusions from.

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