Help: Clarify and correct wording around DEPFILE option

The previous wording contradicted itself regarding whether
Visual Studio generators were supported, and about when
generator expressions could be used. Restructure the paragraphs
and max it clearer what support was added in which CMake
versions.

Help/command/add_custom_command.rst

@@ -271,35 +271,40 @@ The options are:
 ``DEPFILE``
   .. versionadded:: 3.7
 
-  Specify a ``.d`` depfile for the :generator:`Ninja`, :generator:`Xcode` and
-  :ref:`Makefile <Makefile Generators>` generators. The depfile may use
-  "generator expressions" with the syntax ``$<...>``. See the
-  :manual:`generator-expressions(7) <cmake-generator-expressions(7)>` manual
-  for available expressions. A ``.d`` file holds dependencies usually emitted
-  by the custom command itself.
+  Specify a ``.d`` depfile which holds dependencies for the custom command.
+  It is usually emitted by the custom command itself.  This keyword may only
+  be used if the generator supports it, as detailed below.
 
-  Using ``DEPFILE`` with other generators than :generator:`Ninja`,
-  :generator:`Xcode` or :ref:`Makefile <Makefile Generators>` is an error.
+  .. versionadded:: 3.7
+    The :generator:`Ninja` generator supports ``DEPFILE`` since the keyword
+    was first added.
+
+  .. versionadded:: 3.17
+    Added the :generator:`Ninja Multi-Config` generator, which included
+    support for the ``DEPFILE`` keyword.
 
   .. versionadded:: 3.20
     Added support for :ref:`Makefile Generators`.
 
+    .. note::
+
+      ``DEPFILE`` cannot be specified at the same time as the
+      ``IMPLICIT_DEPENDS`` option for :ref:`Makefile Generators`.
+
   .. versionadded:: 3.21
     Added support for :ref:`Visual Studio Generators` with VS 2012 and above,
-    for the :generator:`Xcode` generator, and for
-    :manual:`generator expressions <cmake-generator-expressions(7)>`.
+    and for the :generator:`Xcode` generator.  Support for
+    :manual:`generator expressions <cmake-generator-expressions(7)>` was also
+    added.
+
+  Using ``DEPFILE`` with generators other than those listed above is an error.
 
   If the ``DEPFILE`` argument is relative, it should be relative to
   :variable:`CMAKE_CURRENT_BINARY_DIR`, and any relative paths inside the
-  ``DEPFILE`` should also be relative to :variable:`CMAKE_CURRENT_BINARY_DIR`
-  (see policy :policy:`CMP0116`. This policy is always ``NEW`` for
+  ``DEPFILE`` should also be relative to :variable:`CMAKE_CURRENT_BINARY_DIR`.
+  See policy :policy:`CMP0116`, which is always ``NEW`` for
   :ref:`Makefile Generators`, :ref:`Visual Studio Generators`,
-  and the :generator:`Xcode` generator).
-
-  .. note::
-
-    For :ref:`Makefile Generators`, this option cannot be specified at the
-    same time as ``IMPLICIT_DEPENDS`` option.
+  and the :generator:`Xcode` generator.
 
 Examples: Generating Files
 ^^^^^^^^^^^^^^^^^^^^^^^^^^