docs updates

README.rst

@@ -134,6 +134,10 @@ Options:
   not set, MBS will try to find a compatible module build to reuse components from.
 - ``scratch`` - a boolean indicating if a scratch module build should be performed.
   Only allowed to be ``True`` if the MBS setting ``MODULES_ALLOW_SCRATCH`` is ``True``.
+- ``side_tag`` - this string value instructs mbs to use a side tag for any required base modules.
+  If a base module has ``koji_side_tag_format`` defined, then that format is evaluated with the
+  value given here to determine the tag to use for that base module.
+  If ``koji_side_tag_format`` is not defined for a base module, then this option has no effect.
 - ``srpms`` - an optional list of Koji upload URLs of SRPMs to include in a module scratch build.
   Only allowed if ``scratch`` is ``True``.
 - ``yaml`` - a string of the input file when submitting a YAML modulemd file directly in a

docs/VIRTUAL_MODULES.rst

@@ -66,6 +66,9 @@ Custom fields in xmd:
 - ``koji_tag`` - this defines the Koji tag with the RPMs that are part of this module. For base
   modules this will likely be a tag representing a buildroot. If this is a metadata-only module,
   then this can be left unset.
+- ``koji_side_tag_format`` - this field is used instead of ``koji_tag`` when the ``side_tag``
+  option is given for a build. In such a case, this field is treated as a python format string
+  and expanded with given value for ``side_tag``.
 - ``koji_tag_with_modules`` - this defines the Koji tag with the module builds. These modules are
   later used to fulfill the build requirements of modules built on against this module. This
   option is used only when ``KojiResolver`` is enabled on the MBS server.