build_policy=never for conan export-pkg

mastering/policies.rst

@@ -21,6 +21,7 @@ With the ``build_policy`` attribute in the `conanfile.py` the package creator ca
 - ``missing``: If no binary package is found, Conan will build it without the need to invoke Conan install with :command:`--build missing`
   option.
 - ``always``: The package will be built always, **retrieving each time the source code** executing the "source" method.
+- ``never``: (experimental) Never builds this package from source, this package can only be created with a ``conan export-pkg`` command.
 
 
 .. code-block:: python

reference/commands/creator/export-pkg.rst

@@ -137,6 +137,12 @@ There are different scenarios where this command could look like useful:
    :ref:`How to package existing binaries <existing_binaries>`.
 
 
+Packages created with ``conan export-pkg`` cannot be rebuilt from sources in the cache with the ``--build``
+command line argument. It is possible to specify the class attribute ``build_policy="never"`` in this recipes
+(this is an experimental feature) to avoid the ``--build=*`` or ``--build`` argument to try to rebuild them from sources
+as part of a dependency graph.
+
+
 .. note::
 
     Note that if :command:`--profile`, settings or options are not provided to :command:`export-pkg`,