[hist] Remove mentions of SetBinContent

We decided to not add this method to the public interface because it
has unclear semantics for a number of cases, in particular related to
global histogram statistics. While it might be fine to allow it in
RHistEngine, it would also be confusing to then not have it in RHist.

Author: hahnjo

hist/histv7/doc/CodeArchitecture.md

@@ -68,7 +68,7 @@ Objects of this type are passed by value.
 
 A single bin index, which is just an integer for normal bins.
 `Underflow()` and `Overflow()` are special values and not ordered with respect to others.
-Objects of this type are passed by value; most notably to `GetBinContent` and `SetBinContent`.
+Objects of this type are passed by value; most notably to `GetBinContent`.
 
 ### `RBinIndexRange`
 

hist/histv7/doc/DesignImplementation.md

@@ -55,10 +55,9 @@ This will copy the arguments, which is fine in this case because `RBinIndex` is
 ### Special Arguments
 
 Special arguments are passed last.
-Examples include
+For example
 ```cpp
 template <typename... A> void Fill(const std::tuple<A...> &args, RWeight w);
-template <std::size_t N> void SetBinContent(const std::array<RBinIndex, N> &indices, const BinContentType &content);
 ```
 The same works for the variadic function templates that will check the type of the last argument.