"secret" inputs for the "Radon" function

"secret" inputs for the "Radon" function

I found out a function with a "secret" input, someone can explain this
issue...? what can be the reason? There are many cases of this thing?
In the documentation of "radon" function, there are two ways to call the
function- the conventional with inputs of I,THETA, (for image and desired
projection angles), and there is also an option to omit THETA, and give
only I as input.
but, in the comments inside radon.m script, there are another part, at the
end, that called "grandfathered syntax" (??), that allow three inputs,
with including also the number of desired projections. And, it really
work!
So, why isn't it documented? It was available somewhen, and
deleted/changed (as sounds from "grandfathered syntax")? so why it still
works?
And, another question about radon.m:
The main part of the function computed by calling another function, called
"radonc". what is it? where is it existing? I didn't find it or any
mention for it anywhere...
Thanks