Add missing docs for soft_write_failure for master/agent setup

We had added the feature for soft_write_failure in 1.5.x, but the documentation
was only updated for puppet apply cases. This patches updates the documentation
for puppet master/agent setups as well.

Signed-off-by: Ken Barber <ken@bob.sh>
Merge pull request #697 from kbarber/maint/1.5.x/soft_write_failure_docs_missing

Add missing docs for soft_write_failure for master/agent setup