Fix model update add documentation (#1)

[pkonneker]

• Add is performed only on set/number

Summary:

Updated documentation for Model.update to better reflect what $ADD actually does.

GitHub linked issue:

Closes #388

Type (select 1):

• Bug fix
• Feature implementation
• Documentation improvement
• Testing improvement

• Test added to report bug (GitHub issue #--- @---)
• Something not listed here

Is this a breaking change? (select 1):

• 🚨 YES 🚨
• No
• I'm not sure

Is this ready to be merged into Dynamoose? (select 1):

• Yes
• No

Are all the tests currently passing on this PR? (select 1):

• Yes
• No

Other:

• I have searched through the GitHub pull requests to ensure this PR has not already been submitted
• I have updated the Dynamoose documentation (if required) given the changes I made
• I have added/updated the Dynamoose test cases (if required) given the changes I made
• I have run npm test from the root of the project directory to ensure all tests continue to pass
• I agree that all changes made in this pull request may be distributed and are made available in accordance with the Dynamoose license
• All of my commits and commit messages are detailed, explain what changes were made, and are easy to follow and understand
• I have confirmed that all my code changes are indented properly using 2 spaces
• I have filled out all fields above

[coveralls]

Pull Request Test Coverage Report for Build 674

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 84.566%

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Totals
Change from base Build 669:	0.0%
Covered Lines:	1866
Relevant Lines:	2132

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💛 - Coveralls

[fishcharlie]

@pkonneker Thanks for this! Will get merged in soon.

[fishcharlie]

LGTM