Although *make* is the command, logically *make install* is the command-to-which-you-add-DESTDIR; I would move the *install* inside <command>.

putting *the number of 512 byte sectors needed (e.g. 5 megabytes is 10240 sectors)* in

Scans better for me and includes an example.

no space before the comma

No, "make install" is not literally the command; "make installworld", "make installkernel", and "make distribution" are, collectively, "the make install commands".

@ian, does something like this sounds better ?
(I split it in several lines intentionally to make it more readable)

but do not forget to add 
<option>DESTDIR=<replaceable>${NFSROOTDOR} </replaceable></option>
when running the 
<command>make installkernel</command>
and 
<command>make installworld</command> 
commands.

s/NFSROOTDOR/NFSROOTDIR/

;-)

No objection to the changes, just one minor nit.
Does the actual command output state "doesn't" or has the text outputs since been changed?

You're completely right, this is an unwanted change.