tell tidy the input is ISO-8859-1 & to not complain so much

docbook output is ISO-8859-1 and tidy just assumes ASCII
without the '-latin1' option you get things like
  line 86 column 13 - Warning: replacing invalid UTF-8 bytes (char. code U+00A9)
where docbook converts this
  <!entity  my-copy "&copy;">
  Copyright &my-copy; 2001-2017 by Privoxy Developers
to this
  Copyright © 2001-2017 by Privoxy Developers
and tidy strips out the copyright character.

with the '-latin1' option tidy leaves the copyright character as is.

options  -q --mute MISSING_ATTRIBUTE --mute TRIM_EMPTY_ELEMENT
just gets rid of verbose messages for things nobody is going to fix