Add documentation for IDT entry #VE (vector 20)

[josephlr]

This exception vector is supported only on Intel processors.

#313 is a good example of how this documentation should look.

This feature is documented in:

• Section 6.15 - Interrupt 20 of the Intel Software Developer’s Manual Volume 3: System Programming Guide
• Also see Section 25.5.7 - Virtualization Exceptions