[BREAKING] Rename truncation strategy to deletion.

[botandrose]

MongoDB doesn't have a truncate command, and the MongoDB command actually being invoked here on the database is named deleteMany, so I think this makes sense.

Will need to add deprecation warnings to 1.99 for this transition.

Also, I fixed up the configuration docs in the README while I was at it.

[codecov]

Codecov Report

Merging #7 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master        #7   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         6           
  Lines           96        89    -7     
=========================================
- Hits            96        89    -7     

Impacted Files	Coverage Δ
lib/database_cleaner/mongo.rb	100.00% <100.00%> (ø)
lib/database_cleaner/mongo/deletion.rb	100.00% <100.00%> (ø)
spec/database_cleaner/mongo/deletion_spec.rb	100.00% <100.00%> (ø)

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