export: Clarify behavior of --lat-longs

[victorlin]

Description of proposed changes

The previous text mentioned built in mappings but did not specify where to find them or specifics around the overriding behavior.

The link to the GitHub-hosted lat_longs.tsv file associated with the installation's version is not perfect since the local version of lat_longs.tsv can be changed (i.e. in a development environment), but this should work for most users.

Related issue(s)

Closes #1714

Checklist

• Automated checks pass

• Check if you need to add a changelog message

• Check if you need to add tests

• Check if you need to update docs

• Checked output locally:

    --lat-longs TSV       Latitudes and longitudes for geography traits. See this file
                          for the format: <https://github.com/nextstrain/augur/blob/27.0.
                          0/augur/data/lat_longs.tsv>. This file provides 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. (default: None)
  

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.72%. Comparing base (19308f0) to head (314722c).
Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1715   +/-   ##
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  Coverage   72.72%   72.72%           
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  Files          79       79           
  Lines        8286     8287    +1     
  Branches     1697     1697           
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+ Hits         6026     6027    +1     
  Misses       1972     1972           
  Partials      288      288           

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