Fixes #258 Add clearer doc for JSON schema's default keyword

[mothslaw]

Pull request checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
• Build was run locally and any changes were pushed
• Lint has passed locally and any fixes were made for failures

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

No mention of default

What is the new behavior?

Added a warning about misinterpreting JSON schema's default keyword. (Also fixed a couple of incorrect quote characters)

Does this introduce a breaking change?

• Yes
• No

Other information

[nhlien93]

yup, just adding the one line about what it actually is used for would be my only comment as well.