[RF] Clarify usage of RooParametricStepFunction

[ynikitenko]

• [x ] Checked for duplicates

Describe the bug

In the documentation of RooParametricStepFunction the last line of the example goes like

RooParametricStepFunction  aPdf = ("aPdf","PSF",*x, 
    *list,limits,nbins);

where x is undefined in that code sample.

Expected behavior

Please write what x in that example is (that is some RooAbsReal).
Could you please also add the usage of this function? As I understand, it gets its values from a histogram (or does it fit a histogram), so it would be nice to introduce that histogram to the example.
I found no references to this class in tutorials.

P.S. I couldn't run this code in ROOT v.6.22/06 because of the error

error: reference to 'list' is ambiguous
list,limits,nbins);
^
ROOT_prompt_3:1:13: note: candidate found by name lookup is '__cling_N54::list'
RooArgList list = new RooArgList("list");
^
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../include/c++/10.2.0/bits/stl_list.h:556:11: note: candidate found by name lookup is 'std::__cxx11::list'
class list : protected _List_base<_Tp, _Alloc>