Add documentation for missing replication script parameters

Reading the help output and the sources of https://github.com/osm2pgsql-dev/osm2pgsql/blob/6d964b7d08bc17ac2240237f00c80a81f8b87708/scripts/osm2pgsql-replication#L485, I missed a few replication script parameters in the documentation. 

I added documentation for:
- `--once`
- `--post-processing`
- `--diff-file`

Author: hiddewie

_manual-v2/A-preparing-input-data.md

@@ -103,7 +103,7 @@ The `-d` parameter tells the replication script which database to connect
 to. The script also supports all other parameters mentioned in the section
 [database connection](#database-connection) including
 [libpq environment variables](https://www.postgresql.org/docs/current/libpq-envars.html).
-The only exception is the '-W' parameter for interactive password prompts.
+The only exception is the `-W` parameter for interactive password prompts.
 When you need to supply a password, always use a
 [pgpass file](https://www.postgresql.org/docs/current/libpq-pgpass.html).
 
@@ -115,7 +115,8 @@ the replication information.
 By default the update server and interval will be set from the file headers,
 for planet dumps minutely updates from the OSM main servers will be used. If
 you want to use a different replication service, use the `--server`
-parameter.
+parameter. You can also use `--osm-file <osm-file>` to retrieve the update 
+server from a data file.
 
 It is safe to repeat initialisation at any time. For example, when you want
 to change the replication service, simply run the init command again with a
@@ -132,15 +133,21 @@ and calls osm2pgsql with the given parameters to apply the changes. Note that
 osm2pgsql-replication makes sure to only fetch a limited amount of data at the
 time to make sure that it does not use up too much RAM. If more data is
 available it will repeat the download and call of osm2pgsql until the database
-is up to date. You can change the amount of data downloaded at once with
-`--max-diff-size`, the default is 500MB.
+is up to date (unless the option `--once` is used). You can change the amount 
+of data downloaded per at once with `--max-diff-size`, the default is 500MB.
 
 Sometimes you need to run additional commands after osm2pgsql has updated the
 database, for example, when you use the expiry function. You can use the
 option `--post-processing` to give osm2pgsql-replication a script it is
 supposed to run after each call to osm2pgsql. Note that if the script fails,
 then the entire update process is considered a failure and aborted.
 
+You can use the option `--diff-file <file>` to specify a file where osm2pgsql-replication
+will save the changes before they are applied to the database.
+
+After the update has finished, osm2pgsql-replication can run a script specified
+by the option `--post-processing <script>`.
+
 ##### Putting it all together with systemd
 
 osm2pgsql-replication works well as a systemd service that keeps your database