[MRG] update the sourmash __init__.py docstring

[ctb]

Adds a better docstring and __all__ to src/sourmash/__init__.py.

Fixes #1233.

Checklist

• 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]

Codecov Report

Merging #1364 (91765d9) into latest (1e96920) will increase coverage by 5.29%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           latest    #1364      +/-   ##
==========================================
+ Coverage   88.84%   94.13%   +5.29%     
==========================================
  Files         123       96      -27     
  Lines       18273    14656    -3617     
  Branches     1409     1409              
==========================================
- Hits        16235    13797    -2438     
+ Misses       1800      621    -1179     
  Partials      238      238              

Flag	Coverage Δ
python	94.13% <100.00%> (-0.01%)	⬇️
rust	?

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Impacted Files	Coverage Δ
src/sourmash/__init__.py	90.90% <100.00%> (-0.76%)	⬇️
src/core/src/sketch/minhash.rs
src/core/src/from.rs
src/core/src/lib.rs
src/core/src/index/search.rs
src/core/tests/minhash.rs
src/core/src/ffi/cmd/compute.rs
src/core/src/errors.rs
src/core/src/ffi/hyperloglog.rs
src/core/src/ffi/mod.rs
... and 18 more

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[ctb]

Ready for review @luizirber @bluegenes

[bluegenes]

is load_one_signature also deprecated / to be removed in 5.0?

[ctb]

no - it's used too widely/is too useful - see #1062 for discussion.

I know it's a little inconsistent but I went for practicality here ;).

ref #1279

[bluegenes]

I don't think I can properly evaluate additional things that might be useful here .. but this seems more informative/useful than before, makes sense, looks good

[ctb]

thanks @bluegenes! @luizirber lmk if you have additional thoughts, no particular hurry on merging this.