Add exceptions to F.15, F.16, and F.18 for shared_ptr types

Currently these guidelines conflict with R.34, R.35, and R.36.

This conflict has led to confusion, where it's unclear which
guidelines to prefer for `shared_ptr` types.

In a [previous
PR](#1989) I proposed
preferring the "F" series of guidelines.  This PR takes the opposite
approach and prefers the "R" guidelines for `shared_ptr` types.

I don't feel strongly about which guidelines to prefer, I just want to make
sure the guidelines are internally consistent.

CppCoreGuidelines.md

@@ -2978,6 +2978,11 @@ Use the advanced techniques only after demonstrating need, and document that nee
 
 For passing sequences of characters see [String](#SS-string).
 
+##### Exception
+
+To express shared ownership using `shared_ptr` types, rather than following guidelines F.16-21,
+instead follow [R.34](#Rr-sharedptrparam-owner), [R.35](#Rr-sharedptrparam), and [R.36](#Rr-sharedptrparam-const).
+
 ### <a name="Rf-in"></a>F.16: For "in" parameters, pass cheaply-copied types by value and others by reference to `const`
 
 ##### Reason
@@ -3029,6 +3034,11 @@ If you need the notion of an optional value, use a pointer, `std::optional`, or
 * (Simple) ((Foundation)) Warn when a parameter passed by reference to `const` has a size less than `2 * sizeof(void*)`. Suggest passing by value instead.
 * (Simple) ((Foundation)) Warn when a parameter passed by reference to `const` is `move`d.
 
+##### Exception
+
+To express shared ownership using `shared_ptr` types, instead follow [R.34](#Rr-sharedptrparam-owner) or [R.36](#Rr-sharedptrparam-const),
+depending on whether or not the function unconditionally takes a reference to the argument.
+
 ### <a name="Rf-inout"></a>F.17: For "in-out" parameters, pass by reference to non-`const`
 
 ##### Reason
@@ -3107,6 +3117,10 @@ For example:
         // use p ... possibly std::move(p) onward somewhere else
     }   // p gets destroyed
 
+##### Exception
+
+If the "will-move-from" parameter is a `shared_ptr` instead follow [R.34](#Rr-sharedptrparam-owner) and pass the `shared_ptr` by value.
+
 ##### Enforcement
 
 * Flag all `X&&` parameters (where `X` is not a template type parameter name) where the function body uses them without `std::move`.