It's documented in fsck(8), but not fsck_*(8). This can confuse people. I just copied its paragraph from fsck(8).
Index: sbin/fsck_ext2fs/fsck_ext2fs.8 =================================================================== RCS file: /cvs/src/sbin/fsck_ext2fs/fsck_ext2fs.8,v retrieving revision 1.16 diff -u -p -r1.16 fsck_ext2fs.8 --- sbin/fsck_ext2fs/fsck_ext2fs.8 10 Jan 2010 10:53:33 -0000 1.16 +++ sbin/fsck_ext2fs/fsck_ext2fs.8 26 Jul 2015 22:55:27 -0000 @@ -166,6 +166,17 @@ this should be used with great caution a to continue after essentially unlimited trouble has been encountered. .El .Pp +If neither of the +.Fl y +or +.Fl n +options are specified, the user may force +.Nm +to assume an answer of +.Dq yes +to all the remaining questions by replying to a question with a value of +.Dq F . +.Pp Inconsistencies checked are as follows: .Pp .Bl -enum -compact Index: sbin/fsck_ffs/fsck_ffs.8 =================================================================== RCS file: /cvs/src/sbin/fsck_ffs/fsck_ffs.8,v retrieving revision 1.23 diff -u -p -r1.23 fsck_ffs.8 --- sbin/fsck_ffs/fsck_ffs.8 11 Feb 2013 17:35:46 -0000 1.23 +++ sbin/fsck_ffs/fsck_ffs.8 26 Jul 2015 22:55:27 -0000 @@ -216,6 +216,17 @@ this should be used with great caution a to continue after essentially unlimited trouble has been encountered. .El .Pp +If neither of the +.Fl y +or +.Fl n +options are specified, the user may force +.Nm +to assume an answer of +.Dq yes +to all the remaining questions by replying to a question with a value of +.Dq F . +.Pp In interactive mode, .Nm will list the conversion to be made Index: sbin/fsck_msdos/fsck_msdos.8 =================================================================== RCS file: /cvs/src/sbin/fsck_msdos/fsck_msdos.8,v retrieving revision 1.14 diff -u -p -r1.14 fsck_msdos.8 --- sbin/fsck_msdos/fsck_msdos.8 16 Jun 2014 18:33:33 -0000 1.14 +++ sbin/fsck_msdos/fsck_msdos.8 26 Jul 2015 22:55:27 -0000 @@ -93,6 +93,17 @@ to assume .Dq yes as the answer to all operator questions. .El +.Pp +If neither of the +.Fl y +or +.Fl n +options are specified, the user may force +.Nm +to assume an answer of +.Dq yes +to all the remaining questions by replying to a question with a value of +.Dq F . .Sh SEE ALSO .Xr fs 5 , .Xr fstab 5 ,
