doc: add an example for variable name :version (#10760)

Fixes #10758

Signed-off-by: Etienne Millon <me@emillon.org>

doc/howto/opam-file-generation.rst

@@ -113,9 +113,10 @@ General Notes and Tips
   keep the corresponding opam fields in a ``pkg.opam.template`` file. See
   :doc:`../reference/packages`.
 - It is not necessary to specify ``(version)``, this will be added at release
-  time if you use `dune-release`_.
-
-.. _dune-release: https://github.com/tarides/dune-release
+  time if you use `dune-release <https://github.com/tarides/dune-release>`_.
+- To generate an opam variable such as ``version``, use a colon ``:`` followed
+  by the name of the variable. For example, to generate ``a { = version }`` in
+  the opam file, use ``(a (= :version))`` in ``dune-project``.
 
 Generating Opam Files
 ---------------------
@@ -130,3 +131,11 @@ If you only want to generate the opam file, run ``dune build <project_name>.opam
 
 Run ``dune build`` once and observe that the opam files have been created or
 updated. Make sure to add these changes to your version control system.
+
+.. seealso::
+
+   :token:`~pkg-dep:dep_specification`
+     How ``(depends)`` and similar fields are processed.
+
+   :doc:`/explanation/opam-integration`
+     How ``with-test`` and related variables are used by opam.