Feature request - document what `get_property $pin activity` returns

[oharboe]

• document all the "source" variants(vcd, user, constant, unknown, clock, internal, etc.). Some of these are obvious, like vcd, but it is less obvious where "constant" comes from and what "internal" means.
• what numbers are being returned, what units are they and the definition of the units

I added some testing code to mock-array to verify that read_vcd was setting activity on all the pins.