[ntuple] Add additional comments related to user-provided ptrs

To squash with #8ea3951.

Author: enirolf

tree/ntuple/inc/ROOT/RNTupleProcessor.hxx

@@ -105,7 +105,7 @@ public:
    }
 
    /////////////////////////////////////////////////////////////////////////////
-   /// \brief Get a raw pointer to the field value managed by the processor's entry.
+   /// \brief Get a non-owning pointer to the field value managed by the processor's entry.
    ///
    /// \return A `T*` if the field is valid in the current entry, or a `nullptr` otherwise.
    T *GetRawPtr() const { return GetPtr().get(); }
@@ -114,6 +114,12 @@ public:
    /// \brief Bind the value to `valuePtr`.
    ///
    /// \param[in] valuePtr Pointer to bind the value to.
+   ///
+   /// \warning Use this function with care! Values may not always be valid for every entry during processing, for
+   /// example when a field is not present in one of the chained processors or when during a join operation, no matching
+   /// entry in the auxiliary processor can be found. Reading `valuePtr` as-is therefore comes with the risk of reading
+   /// invalid data. After binding a pointer to an `RNTupleProcessorOptionalPtr`, we *strongly* recommend only accessing
+   /// its data through this interface, to ensure that only valid data can be read.
    void BindRawPtr(T *valuePtr) { fProcessorEntry->BindRawPtr(fFieldIndex, valuePtr); }
 
    /////////////////////////////////////////////////////////////////////////////
@@ -182,7 +188,7 @@ public:
    }
 
    /////////////////////////////////////////////////////////////////////////////
-   /// \brief Get a raw pointer to the field value managed by the processor's entry.
+   /// \brief Get a non-owning pointer to the field value managed by the processor's entry.
    ///
    /// \return A `void*` if the field is valid in the current entry, or a `nullptr` otherwise.
    void *GetRawPtr() const { return GetPtr().get(); }
@@ -191,6 +197,12 @@ public:
    /// \brief Bind the value to `valuePtr`.
    ///
    /// \param[in] valuePtr Pointer to bind the value to.
+   ///
+   /// \warning Use this function with care! Values may not always be valid for every entry during processing, for
+   /// example when a field is not present in one of the chained processors or when during a join operation, no matching
+   /// entry in the auxiliary processor can be found. Reading `valuePtr` as-is therefore comes with the risk of reading
+   /// invalid data. After binding a pointer to an `RNTupleProcessorOptionalPtr`, we *strongly* recommend only accessing
+   /// its data through this interface, to ensure that only valid data can be read.
    void BindRawPtr(void *valuePtr) { fProcessorEntry->BindRawPtr(fFieldIndex, valuePtr); }
 };
 
@@ -374,6 +386,12 @@ public:
    /// \param[in] fieldName Name of the requested field.
    ///
    /// \return An RNTupleProcessorOptionalPtr, which provides access to the field's value.
+   ///
+   /// \warning Provide a `valuePtr` with care! Values may not always be valid for every entry during processing, for
+   /// example when a field is not present in one of the chained processors or when during a join operation, no matching
+   /// entry in the auxiliary processor can be found. Reading `valuePtr` as-is therefore comes with the risk of reading
+   /// invalid data. After passing a pointer to `RequestField`, we *strongly* recommend only accessing its data through
+   /// the interface of the returned `RNTupleProcessorOptionalPtr`, to ensure that only valid data can be read.
    template <typename T>
    RNTupleProcessorOptionalPtr<T> RequestField(std::string_view fieldName, void *valuePtr = nullptr)
    {