Docs: Avoid use of 'default: null' (#45795)

Various modules document the default 'null' value, but it causes None to
be shown in the documentation explicitly.

lib/ansible/modules/packaging/os/dnf.py

@@ -77,9 +77,7 @@ options:
     description:
       - Specifies an alternative release from which all packages will be
         installed.
-    required: false
     version_added: "2.6"
-    default: null
 
   autoremove:
     description:
@@ -87,7 +85,7 @@ options:
         installed as dependencies of user-installed packages but which are no longer
         required by any such package. Should be used alone or when state is I(absent)
     type: bool
-    default: false
+    default: "no"
     version_added: "2.4"
   exclude:
     description:
@@ -112,7 +110,6 @@ options:
     description:
       - When using latest, only update installed packages. Do not install packages.
       - Has an effect only if state is I(latest)
-    required: false
     default: "no"
     type: bool
     version_added: "2.7"
@@ -125,7 +122,6 @@ options:
   bugfix:
     description:
       - If set to C(yes), and C(state=latest) then only installs updates that have been marked bugfix related.
-    required: false
     default: "no"
     type: bool
     version_added: "2.7"
@@ -133,13 +129,11 @@ options:
     description:
       - I(Plugin) name to enable for the install/update operation.
         The enabled plugin will not persist beyond the transaction.
-    required: false
     version_added: "2.7"
   disable_plugin:
     description:
       - I(Plugin) name to disable for the install/update operation.
         The disabled plugins will not persist beyond the transaction.
-    required: false
     version_added: "2.7"
   disable_excludes:
     description:
@@ -147,7 +141,6 @@ options:
       - If set to C(all), disables all excludes.
       - If set to C(main), disable excludes defined in [main] in yum.conf.
       - If set to C(repoid), disable excludes defined for given repo id.
-    required: false
     choices: [ all, main, repoid ]
     version_added: "2.7"
   validate_certs:
@@ -168,19 +161,18 @@ options:
         package and others can cause changes to the packages which were
         in the earlier transaction).
     type: bool
-    default: False
+    default: "no"
     version_added: "2.7"
   install_repoquery:
     description:
       - This is effectively a no-op in DNF as it is not needed with DNF, but is an accepted parameter for feature
         parity/compatibility with the I(yum) module.
     type: bool
-    default: True
+    default: "yes"
     version_added: "2.7"
   download_only:
     description:
       - Only download the packages, do not install them.
-    required: false
     default: "no"
     type: bool
     version_added: "2.7"