Some minor tweaks:

• Refer to 386 CPUs as 80386 instead of i386 since i386 is commonly used as the architecture name rather than a specific processor.
• Clarify that 5.x also supports 80386's, just not in GENERIC.
• Note that SMP only works on Pentium-class CPUs and higher (we don't support systems with multiple 486's and external APICs). Also, tweak the wording to avoid saying 'supported' twice in a row.