Document scalability spillover risk during upgrades

[monrog2]

Summary

• clarify that a capacity TCA does not necessarily indicate a current outage
• explain how upgrade-driven resource redistribution can create spillover beyond supported capacity
• direct operators to restore sufficient capacity headroom before upgrading

Validation

• /Users/gmonroy/Library/Python/3.9/bin/mkdocs build --strict -f docs/mkdocs.yml

Closes #427