Docs/Packaging guide: Add BundlePackage (#39691)

* Docs/Packaging guide: Add BundlePackage
* Adjusted version ordering note to reflect convention.

lib/spack/docs/build_systems/bundlepackage.rst

@@ -9,9 +9,32 @@
 Bundle
 ------
 
-``BundlePackage`` represents a set of packages that are expected to work well
-together, such as a collection of commonly used software libraries.  The
-associated software is specified as bundle dependencies.
+``BundlePackage`` represents a set of packages that are expected to work
+well together, such as a collection of commonly used software libraries.
+The associated software is specified as dependencies.
+
+If it makes sense, variants, conflicts, and requirements can be added to
+the package. :ref:`Variants <variants>` ensure that common build options
+are consistent across the packages supporting them.  :ref:`Conflicts
+and requirements <packaging_conflicts>` prevent attempts to build with known
+bugs or limitations.
+
+For example, if ``MyBundlePackage`` is known to only build on ``linux``,
+it could use the ``require`` directive as follows:
+
+.. code-block:: python
+
+    require("platform=linux", msg="MyBundlePackage only builds on linux")
+
+Spack has a number of built-in bundle packages, such as:
+
+* `AmdAocl <https://github.com/spack/spack/blob/develop/var/spack/repos/builtin/packages/amd-aocl/package.py>`_
+* `EcpProxyApps <https://github.com/spack/spack/blob/develop/var/spack/repos/builtin/packages/ecp-proxy-apps/package.py>`_
+* `Libc <https://github.com/spack/spack/blob/develop/var/spack/repos/builtin/packages/libc/package.py>`_
+* `Xsdk <https://github.com/spack/spack/blob/develop/var/spack/repos/builtin/packages/xsdk/package.py>`_
+
+where ``Xsdk`` also inherits from ``CudaPackage`` and ``RocmPackage`` and
+``Libc`` is a virtual bundle package for the C standard library.
 
 
 ^^^^^^^^