docs/manual: pass PARALLEL_JOBS to NINJA_OPTS

Ninja understands the `-j` option which defines how many jobs are
run in parallel.

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
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