Porter's Handbook: Document DISABLE_CONFLICTS variable

Requested by:	linimon
Reviewed by:    crees
Approved by:    doc (ygy)
Differential Revision:  https://reviews.freebsd.org/D28582

documentation/content/en/books/porters-handbook/makefiles/chapter.adoc

@@ -4631,6 +4631,13 @@ The first entry match versions `2.0` through `2.3`, the second all the revisions
 package:deskutils/gnotime[] does not have any conflicts line because its current version does not conflict with anything else.
 ====
 
+The variable `DISABLE_CONFLICTS` may be temporarily set when making targets that are not affected by conflicts.  The variable is not to be set in port Makefiles.
+
+[source,bash]
+....
+% make -DDISABLE_CONFLICTS patch
+....
+
 [[install]]
 == Installing Files