export: Clarify behavior of --lat-longs

[victorlin]

from #1465 (comment)

Context

The help text for --lat-longs currently says:

Latitudes and longitudes for geography traits (overrides built in mappings)

This confused me in #1465, since it sounds like specifying any file will throw out all the default mappings provided by augur/data/lat_longs.tsv. As mentioned later in that issue, that is not the case.

Proposed solution

Update the help text to something along the lines of:

Latitudes and longitudes for geography traits. See the file augur/data/lat_longs.tsv for the format. This is also the default set of latitudes and longitudes. An additional file specified by this option will extend the default set. Duplicates based on the first two columns will be resolved by taking the coordinates from the user-provided file.

We may want to provide a link to the GitHub-hosted file (https://github.com/nextstrain/augur/blob/-/augur/data/lat_longs.tsv) since we shouldn't expect users to find it in their Augur installation.

[joverlee521]

We may want to provide a link to the GitHub-hosted file (https://github.com/nextstrain/augur/blob/-/augur/data/lat_longs.tsv)

If we do include the link, we might want to point the specific Augur version instead of the default branch since the contents of the file can differ between versions.