documentation update from docuthon

[ctb]

cc @brooksph @taylorreiter

• Is it mergeable?
• make test Did it pass the tests?
• make coverage Is the new code covered?
• Did it change the command-line interface? Only additions are allowed
  without a major version increment. Changing file formats also requires a
  major version number increment.
• Was a spellchecker run on the source code and documentation after
  changes were made?

[codecov-io]

Codecov Report

Merging #172 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #172      +/-   ##
==========================================
+ Coverage   85.52%   85.57%   +0.05%     
==========================================
  Files          13       13              
  Lines        1879     1886       +7     
  Branches       52       52              
==========================================
+ Hits         1607     1614       +7     
  Misses        263      263              
  Partials        9        9

Impacted Files	Coverage Δ
sourmash_lib/__main__.py	100% <ø> (ø)	⬆️
sourmash_lib/sourmash_args.py	93.82% <100%> (+0.58%)	⬆️
sourmash_lib/commands.py	89.44% <100%> (ø)	⬆️

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[betatim]

accidental duplication with add_construct_moltype_args and add_moltype_args? I can't spot the difference :-/

[ctb]

Closed in favor of #240.