Clarify migrate-logs usage #404

Nit: dropping "the" would be more idiomatic.

This wording kind of makes it sound like there are multiple soju databases with different purposes.

The point of using "the" was to emphasize that there is only one database. The original wording in the usage led me to initially think that the there would be a separate database for the messages.

Hmm, looking at this again, I'm now not sure which "the" is being referred to here.

location doesn't work very well for PostgreSQL. source is the wording used in the man page.

It's a bit confusing with <source logs> being something entirely different.

I agree that it would be best to use language that is consistent with the man page, but unfortunately in this context doing so is somewhat misleading. For migrate-logs the database is really a sink, not a source (not 100% true since it extracts some user data).
In my mind I think a better word might be "store", but I'm not sure if you are interested in having such a change to all the documentation.

The intent of the original wording is that the source part of the database string would be "the string that would be in the Soju config file". Here it's switching the meaning to <source logs>, and leaves source/location undefined.

Maybe we could give an example with the original and desired message-store directives from the config file, and the CLI arguments for the script. Something like:

To switch from this old message store configuration:

db sqlite3 /var/lib/soju/main.db
message-store fs /var/lib/soju/messages

To this new configuration:

db sqlite3 /var/lib/soju/main.db
message-store db

The following command line can be used:

migrate-logs /var/lib/soju/messages sqlite3:/var/lib/soju/main.db

I like this, it makes the example very clear.