-@code{-j} command-line option of @command{make} is unsupported for
-building the documentation. As the most time consuming task is
-running LilyPond to build images of music, the makefile variable
-@code{CPU_COUNT} may be set in @file{local.make} or on the command line
-to the number of @code{.ly} files that LilyPond should process
-simultaneously, e.g. on a bi-processor or dual core machine
+The most time consuming task for building the documentation is running
+LilyPond to build images of music, and there cannot be several
+simultaneously running @command{lilypond-book} instances, so @code{-j}
+@command{make} option does not significantly speed up the build process.
+To help speed it up, the makefile variable @var{CPU_COUNT} may be set
+in @file{local.make} or on the command line to the number of
+@code{.ly} files that LilyPond should process simultaneously, e.g. on
+a bi-processor or dual core machine