docs: clarify backport commands and behavior

[joshtrichards]

Mostly because of #854, but some very very minor tweaking while I was in the file.

Summary

• Clarify who can request backports (once fix: backport author association policy and align checks #854 is merged)
• Consolidate/tighten up the supported /backport command formats and examples
• Document "how it works" a bit more extensively:
  • Standard versus forced (/backport!) backport behavior
  • Conflict handling and backport intermediate branches
• Adding missing bot reaction

🤖 AI (if applicable)

• The content of this PR was partly or fully generated using AI