You need to keep the "The" at the start of the sentence.

Man page style has the new sentence start on a new line.

It is probably more technically correct to say that MAKEOBJDIRPREFIX is set in make's environment; just setting an unexported shell variable would be local to the shell; setting it on the make command line works for make but is treated slightly differently than inheriting it from the shell environment.
(The same discussion applies to the next chunk.)

Start this sentence with "The" as well.
Also, "build location" is not exactly a well-defined concept; it may be better to use more words and say that "the location of the object files generated by this build can be changed from the default..."

I wanted to quickly remind users that the variable can be set to change the build output directory, similar to installworld and installkernel does. This acts as a quick reminder in case people are jumping straight to these sections instead of reading the whole man page. Yes RTFM is a separate discussion :) There is already text in the beginning of the man page and in the environment section that talks more in detail about all the variables.

I did change it to make variable as the rest of the text references. And changed build location to object directory.

See reply above.

"directory" is redundant here.

.Xr make 1
variable.

"directory" is redundant here, too.

.Xr make 1
variable.