Update documentation of pstore

After systemd has started to support pstore the documentation needs to be adapted accordingly.

Upon startup, systemd-pstore will monitor and extract the pstore contents and save it inside /var/lib/systemd/pstore/.

This MR is related to tests/apertis-test-cases!603 (merged)

Signed-off-by: Nithin M N MN.Nithin@in.bosch.com