DOC: internals: cleanup few typos in master-worker documentation

DOC: internals: cleanup few typos in master-worker documentation

s/mecanism/mechanism
s/got ride/got rid
s/traditionnal/traditional

One typo is confusion between master and worker that results to a
semantic mistake in the sentence:

"...the master will emit an "exit-on-failure" error and will kill every
workers with a SIGTERM and exits with the same error code than the
failed [-master-]{+worker+}..."

Should be backported as far as 3.1.