doc: Add statment about permissions needed to do a core dump

Documentation did not specify, that some permissions are required on
target path for coredump for the user running the hypervisor.

Diff to v1:
- reword statements

src/libvirt.c

@@ -2777,7 +2777,8 @@ error:
  *
  * This method will dump the core of a domain on a given file for analysis.
  * Note that for remote Xen Daemon the file path will be interpreted in
- * the remote host.
+ * the remote host. Hypervisors may require  the user to manually ensure
+ * proper permissions on the file named by @to.
  *
  * If @flags includes VIR_DUMP_CRASH, then leave the guest shut off with
  * a crashed state after the dump completes.  If @flags includes

tools/virsh.pod

@@ -621,6 +621,9 @@ rather than merely left in a paused state.
 If I<--bypass-cache> is specified, the save will avoid the file system
 cache, although this may slow down the operation.
 
+NOTE: Some hypervisors may require the user to manually ensure proper
+permissions on file and path specified by argument I<corefilepath>.
+
 =item B<dumpxml> I<domain-id> [I<--inactive>] [I<--security-info>]
 [I<--update-cpu>]