Use stricter multi-document properties separator logic

[philwebb]

See https://twitter.com/genzerhawker/status/1294419849339260929

Is the `#----` marker fixed or can be changed somehow? I have several properties files with

#----
# GROUP OF CONFIGS
#----

Will this practice make S.B treat a properties file as a multi-document one?

[philwebb]

We could perhaps check that the previous and next lines are not comments. That would reduce the chance of false positives.