docs..MultiplexConfig: Explain the merging of multiple files

It might not be obvious how merging multiple yaml files work. Here is a
simple explanation.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

docs/source/MultiplexConfig.rst

@@ -67,7 +67,15 @@ The ending nodes (the leafs on the tree) will become part of all lower-level
 However, the precedence is evaluated in top-down or ``last defined`` order.
 In other words, the last parsed has precedence over earlier definitions.
 
-It's also possible to remove nodes using python regular
+When you provide multiple files they are processed and merged together using
+the common root (`/`). When certain paths overlap (`$file1:/my/path`,
+`$file2:/my/path`), we first create the tree of `$file1` and then process
+`$file2`. This means all children of `/my/path` of the first file are in
+correct order and `$file2` either updates values or appends new children
+as next ones. This of course happens recursively so you update valures and add
+children of all the nodes beneath.
+
+During this merge it's also possible to remove nodes using python regular
 expressions, which can be useful when extending upstream file using downstream
 yaml files. This is done by `!remove_node : $value_name` directive::