MDEV-22980: Allow plugin-maturity=alpha so S3 plugin loads

Verified by running before and after:

  mariadb --skip-column-names -e "select plugin_name, plugin_status,
  plugin_type, plugin_library, plugin_license from
  information_schema.all_plugins order by plugin_name, plugin_library"

Nothing else but exactly this line changed so there are no side effects:

  -S3 NOT INSTALLED	STORAGE ENGINE	ha_s3.so	GPL
  +S3 ACTIVE	       STORAGE ENGINE	ha_s3.so	GPL

Also enrich config file with link to KB and unify option syntax and
standard comments.

storage/maria/s3.cnf

+[mariadbd]
+#
+# Uncomment line to enable
+#
+#plugin-maturity = alpha
+
 [mariadb]
-plugin-load-add=ha_s3
+#
+# Uncomment line to enable
+#
+#plugin-load-add = ha_s3
 
 #
-# Uncomment and configure the S3 engine
+# Uncomment to configure the S3 engine
+# See all options at https://mariadb.com/kb/en/s3-storage-engine/
 #
-#s3-host-name=s3.amazonaws.com
-#s3-protocol-version=Amazon
-#s3-bucket=...
-#s3-access-key=...
-#s3-secret-key=...
-#s3-region=eu-north-1
+#s3-host-name = s3.amazonaws.com
+#s3-protocol-version = Amazon
+#s3-bucket = ...
+#s3-access-key = ...
+#s3-secret-key = ...
+#s3-region = eu-north-1