docs(error-messages): clarify guidelines for error message text

What does this MR do?

This MR clarifies the guidelines around the usage of an overly friendly tone of voice in error messages, provides an example of what not to do.

This MR originates from this Slack thread.

Does this MR meet the acceptance criteria?

• The MR title and commit messages meet the Pajamas commit conventions.
• [ x The “What does this MR do?” section in the MR description is filled out, explaining the reasons for and scope of the proposed changes, per “Say why not just what”.
  • For example, if the MR is focused on usage guidelines, addressing accessibility challenges could be added in a separate MR.
• Relevant label(s) are applied to the MR.
• The MR is added to a milestone.
• If creating a new component page from scratch, it follows the page template structure.
• Content follows the Pajamas voice and tone guidelines, falling back on the GitLab Documentation Style Guide when needed.
• Related pages are cross-linked, where helpful. Component pages have related components and patterns defined in their Markdown front matter.
• If embedding a Figma file, it follows the Figma embed guide.
• Review requested from any GitLab designer or directly from a maintainer or trainee maintainer.

Links