Migration Guide: document best practices, managed domain, performance implications, ...

The Migration Guide lacks some content about our "migration philosophy". Chapter 2 is a great start, but it should have more content about:

• best practices (e.g. "try migration in a staging environment before migrating prod", "conduct a performance test", how to migrate a cluster etc.; some of these are probably obvious, some of them less so)
• treatment of managed domain (imagine a managed domain with 100s of hosts – how does one approach a gradual migration of such environment)
• performance implications – a short chapter that links to Performance Guide, once we have it (I think this boils down to Undertow[1], other technologies in EAP 7 probably don't have such different performance profile compared to EAP 6. Also, there might be small differences e.g. in memory footprint, startup times etc.)
• migrating EAP installed from RPMs
• migrating EAP that is running as a service

[1] Undertow is a completely new HTTP server with a different performance profile compared to JBoss Web. JBoss Web was a traditional synchronous web server with a thread per request. Undertow uses an asynchronous architecture, which scales better and handles much higher load. This typically comes with a cost of slightly higher response times. But I'm really no expert on Undertow to authoritatively claim how it performs; this paragraph is just common, generic knowledge.