documentation: added information about allowed characters for filenames

doc/btrbk.conf.5

@@ -37,6 +37,10 @@ ssh-url instead of a local directory. The syntax for \fI<url>\fR is:
 .IP
 ssh://host.xz/path/to/volume
 .PP
+Note that btrfs is very picky on file names (mainly for security
+reasons), only the characters [0-9] [a-z] [A-Z] and "._-@" are
+allowed.
+.PP
 The configuration options are:
 .TP
 \fBsnapshot_dir\fR  <directory>