[MRG] Add some more doctests for API definition.

[ctb]

Fixes #329 #147 by adding lots of API documentation.

See expanded docs here.

Also fixes #580 (database scaled values) 'cause why not.

• 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 #436 into master will increase coverage by 0.1%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master     #436     +/-   ##
=========================================
+ Coverage   88.53%   88.64%   +0.1%     
=========================================
  Files          25       25             
  Lines        3786     3786             
  Branches       37       37             
=========================================
+ Hits         3352     3356      +4     
+ Misses        434      430      -4

Impacted Files	Coverage Δ
sourmash/signature.py	95.34% <0%> (+0.58%)	⬆️
sourmash/sbtmh.py	85.29% <0%> (+2.2%)	⬆️

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[ctb]

I think another pass is needed to adjust the language better in the num vs scaled section, but otherwise I like. @luizirber any thoughts?

[ctb]

I've decided I like it. @luizirber ready for review and merge? :)

[luizirber]

I think it does a good job at covering what we have nowadays.

What bothers me is that we still conflate sigs and minhashes sometimes (especially the load_one_signature function), but that will only be fixable in sourmash 3.0...