describe begin/end markers and .pve-ignore.

Added the description right between the list of the changes
we make and the paragraph about how they are os specific
which contains the bit about unmanaged containers which I'm
also making a bit more explicit.

pct.adoc

@@ -205,8 +205,27 @@ rewrite ssh_host_keys:: so that each container has unique keys
 
 randomize crontab:: so that cron does not start at the same time on all containers
 
-The above task depends on the OS type, so the implementation is different
-for each OS type. You can also disable any modifications by manually
+Changes made by {PVE} are enclosed by comment markers:
+
+ # --- BEGIN PVE ---
+ <data>
+ # --- END PVE ---
+
+If no such section is found it will be inserted at a reasonable location
+in the file.
+
+If such a section already exists it will be updated in place and will not be
+moved.
+
+Modification of a file can be prevented by adding a `.pve-ignore.` file for it.
+For instance, if the file `/etc/.pve-ignore.hosts` exists then the
+`/etc/hosts` file will not be touched. (This can be a simple empty file creatd
+via:
+
+ # touch /etc/.pve-ignore.hosts
+
+The above tasks are OS dependent and so they differ between different
+distributions. You can also disable all modifications entirely by manually
 setting the 'ostype' to 'unmanaged'.
 
 OS type detection is done by testing for certain files inside the