doc: Add versions when opam-file related stanzas were added (#10984)

There is no note about `generate_opam_files` in the changelog so the
only reference seems to be the source.

Signed-off-by: Marek Kubica <marek@tarides.com>

doc/reference/dune-project/generate_opam_files.rst

@@ -3,6 +3,8 @@ generate_opam_files
 
 .. describe:: (generate_opam_files ...)
 
+   .. versionadded:: 1.10
+
    Use metadata specified in the ``dune-project`` file to generate ``.opam``
    files.
 
@@ -20,6 +22,8 @@ defined in the project:
 
 .. describe:: (license <strings>)
 
+   .. versionadded:: 1.9
+
    Specify the license of the project, ideally as an identifier from the `SPDX
    License List <https://spdx.org/licenses/>`__.
 
@@ -33,6 +37,8 @@ defined in the project:
 
 .. describe:: (authors <strings>)
 
+   .. versionadded:: 1.9
+
    Specify authors.
 
    Example:
@@ -45,6 +51,8 @@ defined in the project:
 
 .. describe:: (maintainers <strings>)
 
+   .. versionadded:: 1.10
+
    Specify maintainers.
 
    Example:
@@ -57,6 +65,8 @@ defined in the project:
 
 .. describe:: (source ...)
 
+   .. versionadded:: 1.7
+
    Specify where the source for the package can be found.
 
    It can be specified as ``(uri <uri>)`` or using shortcuts for some
@@ -92,6 +102,8 @@ defined in the project:
 
 .. describe:: (bug_reports <url>)
 
+   .. versionadded:: 1.10
+
    Where bugs should be reported.
 
    If a hosting service is used in ``(source)``, a default value is provided.
@@ -104,6 +116,8 @@ defined in the project:
 
 .. describe:: (homepage <url>)
 
+   .. versionadded:: 1.10
+
    The homepage of the project.
 
    If a hosting service is used in ``(source)``, a default value is provided.
@@ -116,6 +130,8 @@ defined in the project:
 
 .. describe:: (documentation <url>)
 
+   .. versionadded:: 1.10
+
    Where the documentation is hosted.
 
 With these fields, every time one calls Dune to execute some rules (either via