Python documentation

[zwimer]

The documentation for python's Extract is incorrect / complete. The args are high, low, a, yet that is only one possible case. There others; in fact the example given simplify(Extract(StringVal("abcd"),2,1)) needs args labeled something like a, start, count.

Related, functions that call extract have incomplete docs. For example, SubString; which shows passing Extract the args s, offset, length. But since this is just a passthrough, high, low, and a work as well. The docs should say this so that if one mistypes the order, it won't bunt the issue down the line for when some other function gets the wrong input since SubString output something unexpected. SubSeq also passes through.

[NikolajBjorner]

I added some more explanation for now