Add description of the valid return values for the fuse_opt_proc_t callback
function that is passed to fuse_opt_parse(3).

ok?


Index: fuse_opt.3
===================================================================
RCS file: /cvs/src/lib/libfuse/fuse_opt.3,v
diff -u -p -r1.8 fuse_opt.3
--- fuse_opt.3	2 Feb 2026 23:35:20 -0000	1.8
+++ fuse_opt.3	11 Mar 2026 12:36:50 -0000
@@ -331,6 +331,15 @@ return 0 on success or \-1 on error.
 .Pp
 .Fn fuse_opt_match
 returns 1 on match or 0 if no match.
+.Pp
+The
+.Fn fuse_opt_proc_t
+callback shall return 1 to retain the option in the output argument vector,
+or 0 to discard it.
+This permits options to be preserved for subsequent processing.
+A return value of \-1 indicates an error condition and causes
+.Fn fuse_opt_parse
+to terminate with failure.
 .Sh EXAMPLES
 .Bd -literal
 #include <err.h>