I just tried to find references to fragmentation among the man pages.  

$ man -k fragmentation
fragmentation: nothing appropriate

But of course indeed there is:

From fsck_ffs(8):
     After successfully correcting a file system, fsck_ffs will print the
     number of files on that file system, the number of used and free
blocks,
     and the percentage of fragmentation.

In fact a more brutal search reveals more:

$ grep -l fragmentation  /usr/share/man/man8/*
/usr/share/man/man8/dhcrelay.8
/usr/share/man/man8/fsck_ffs.8
/usr/share/man/man8/fsck_lfs.8
/usr/share/man/man8/mfs.8
/usr/share/man/man8/mount_mfs.8
/usr/share/man/man8/newfs.8
/usr/share/man/man8/tcpdump.8
/usr/share/man/man8/traceroute.8
/usr/share/man/man8/tunefs.8

but nothing I found in /usr/share/misc/mdoc.template, "man man", or "man
mdoc" tells me what a "keyword" is, such that "man -k" will find it.  

What am I missing, please?  

It seems like a reasonable thing, to want to know, "What's the utility
that reports the fragmentation of my filesystem?"  Wouldn't you say?  

--jkl