moved comment description to first lesson

[josemunozc]

In relation to issue #867

I just moved the description verbatim (and associated key point) to the first lesson and put it after the python example. Another option could be to put the description in brackets right before the first use of #:

This means that assigning a value to one variable does not change values of other variables. For example, let’s store the subject’s weight in pounds in its own variable (everything in a line of code following the ‘#’ symbol is a comment that is ignored by Python, comments allow programmers to leave explanatory notes for other programmers or their future selves):

But might be too lengthy?