Mention that setting item to None in compat is supported (#4278)

Author: AbrilRBS

reference/binary_model/custom_compatibility.rst

@@ -1,3 +1,5 @@
+.. _reference_binary_model_custom_compatibility:
+
 Customizing the binary compatibility
 ====================================
 

reference/conanfile/methods/compatibility.rst

@@ -48,4 +48,5 @@ The format of the list returned is as shown below:
 
 .. seealso::
 
-    Read the :ref:`binary model reference<reference_binary_model>` for a full view of the Conan binary model.
+   - Read the :ref:`binary model reference<reference_binary_model>` for a full view of the Conan binary model.
+   - See more about :ref:`customizing the binary compatibility of your packages<reference_binary_model_custom_compatibility>`

reference/extensions/binary_compatibility.rst

@@ -23,7 +23,7 @@ The following would be valid syntax (but not an useful or working one, as it wil
         return result
 
 
-Conan provides a default ``compatibility.py`` implementation that implements binary compatibility for
+Conan provides a default ``compatibility.py`` that implements binary compatibility for
 different ``compiler.cppstd`` and ``compiler.cstd`` values. That is, by default it assumes that binaries built with different ``cppstd``
 and ``cstd`` values (for the same compiler and compiler version) are binary compatible, and can be linked together
 without issues.
@@ -38,11 +38,13 @@ recipe the ``extension_properties = {"compatibility_cppstd": False}`` attribute,
 From Conan 2.4, the ``compiler.cstd`` setting is available. It will only be taken into account in the computation of
 packages ``package_id`` when their recipes explicitly declare the ``languages = "C"`` attribute.
 
-Some important rules:
-
-- The built-in ``compatibility.py`` is subject to changes in future releases. To avoid being updated
-  in the future, please remove the first comment ``# This file was generated by Conan.``
+There are some cases where the default ``compatibility.py`` will not be enough, and users will need to
+customize it to their needs. Some rules and tips for that customization are explained below.
 
+- Importantly, the built-in ``compatibility.py`` **is subject to changes in future releases**.
+  To avoid being updated in the future, please remove the first comment ``# This file was generated by Conan.``
+- Removing values from ``settings`` or ``options`` in the returned list is possible when they allow being unset,
+  by specifying the value of the item to ``None``.
 
 .. warning::