sin(alpha) documentation issues for some cylinder types

[richardh]

Noted that for some cylinder type models the docs were misleading or even wrong in a couple of places.
I(Q) = integral 0 to pi/2 of F²(Q,alpha)sin(alpha)dalpha

where alpha is angle between rod axis and Q
but usual change of variable u=cos(alpha) means the code does
I(Q) = integral 0 to 1 of F²(Q,u)du
[where strictly we then must do e.g. sin(alpha)=sqrt(1-u²), ]
though some text books also give just this form!

I am making some corections and improvements.

[richardh]

Have also changed orientation angle of rod to Q, called theta in some documentation, to alpha to avoid confusion with the new "orientation angle of rod axis to incident beam direction" theta!