find.1: Fix mandoc warnings and mention a useful manpage

- new sentence, new line
- unknown AT&T UNIX version: At v7
- no blank before trailing delimiter
- reference the ASCII(8) manual page

MFC after:	5 days

usr.bin/find/find.1

@@ -175,7 +175,9 @@ because
 .Ql .\&
 goes before
 .Ql /
-in ASCII. In locales other than
+in
+.Xr ASCII 8 .
+In locales other than
 .Ar C
 results may vary more due to collation differences.
 .It Fl x
@@ -795,7 +797,7 @@ is specified, the given expression shall be effectively replaced by
 .It Ic -print0
 This primary always evaluates to true.
 It prints the pathname of the current file to standard output, followed by an
-.Tn ASCII
+.Xr ASCII 8
 .Dv NUL
 character (character code 0).
 .It Ic -prune