Clarify documentation for isTrue, isFalse

`BasicRubyObject.isTrue` and `.isFalse` correspond to whether the RubyObject is
truthy or falsy, not just whether or not it is an instance of TrueClass or
FalseClass. This documentation change makes that clear.

I confirmed this by calling `isTrue` on a RubyString, which returned true.
Use common spelling of falsey

Metric: rspec, google search result count for: `ruby "falsey"` and `ruby "falsy"`
Merge pull request #1574 from nilbus/patch-1

Clarify documentation for isTrue, isFalse