Improve detail of resolve conflict docs

What does this MR do?

Adds greater detail to the resolve merge request conflict documentation to address the significant confusion about GitLab's approach to merge conflicts, which is different to GitHub.

Related issues

Relates to https://gitlab.com/gitlab-org/gitlab-ce/issues/35054#note_99224224

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Feature moving tiers? Make sure the change is also reflected in features.yml
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards