Help: Expand discussion of GENERATED / BYPRODUCTS

- Mention the Makefile Generators' `make clean` removal in the
  BYPRODUCTS section of add_custom_command and add_custom_target
- Expand the GENERATED property docs' description of which files will
  be marked with the property, and of what it implies (including
  `make clean` removal)

Help/command/add_custom_command.rst

@@ -68,6 +68,9 @@ The options are:
   order-only dependencies to ensure the byproducts will be
   available before their dependents build.
 
+  The :ref:`Makefile Generators` will remove ``BYPRODUCTS`` and other
+  :prop_sf:`GENERATED` files during ``make clean``.
+
 ``COMMAND``
   Specify the command-line(s) to execute at build time.
   If more than one ``COMMAND`` is specified they will be executed in order,

Help/command/add_custom_target.rst

@@ -49,6 +49,9 @@ The options are:
   order-only dependencies to ensure the byproducts will be
   available before their dependents build.
 
+  The :ref:`Makefile Generators` will remove ``BYPRODUCTS`` and other
+  :prop_sf:`GENERATED` files during ``make clean``.
+
 ``COMMAND``
   Specify the command-line(s) to execute at build time.
   If more than one ``COMMAND`` is specified they will be executed in order,

Help/prop_sf/GENERATED.rst

@@ -4,16 +4,29 @@ GENERATED
 Is this source file generated as part of the build or CMake process.
 
 Tells the internal CMake engine that a source file is generated by an outside
-process such as another build step, or the execution of CMake itself. This
-information is then used to exempt the file from any existence or validity
-checks. Generated files are created by the execution of commands such as
-:command:`add_custom_command` and :command:`file(GENERATE)`.
-
-When a generated file created by an :command:`add_custom_command` command
-is explicitly listed as a source file for any target in the same
-directory scope (which usually means the same ``CMakeLists.txt`` file),
-CMake will automatically create a dependency to make sure the file is
-generated before building that target.
+process such as another build step, or the execution of CMake itself.
+This information is then used to exempt the file from any existence or
+validity checks.
+
+Any file that is
+
+- created by the execution of commands such as
+  :command:`add_custom_command` and :command:`file(GENERATE)`
+- listed as one of the ``BYPRODUCTS`` of an :command:`add_custom_command`
+  or :command:`add_custom_target` command, or
+- created by a CMake ``AUTOGEN`` operation such as :prop_tgt:`AUTOMOC`,
+  :prop_tgt:`AUTORCC`, or :prop_tgt:`AUTOUIC`
+
+will be marked with the ``GENERATED`` property.
+
+When a generated file created as the ``OUTPUT`` of an
+:command:`add_custom_command` command is explicitly listed as a source file
+for any target in the same directory scope (which usually means the same
+``CMakeLists.txt`` file), CMake will automatically create a dependency to
+make sure the file is generated before building that target.
+
+The :ref:`Makefile Generators` will remove ``GENERATED`` files during
+``make clean``.
 
 Generated sources may be hidden in some IDE tools, while in others they might
 be shown. For the special case of sources generated by CMake's :prop_tgt:`AUTOMOC`