Enhanced RDoc for CSV::Table

[BurdetteLamar]

Methods:

• to_a
• to_csv
• inspect

Also:

• Tuned up some of the call-seq.
• Removed references to *-convertible, per new guide: https://github.com/ruby/ruby/blob/master/doc/method_documentation.rdoc#argument-description-if-necessary-

(I know, I've violated the one-topic principle.)

[BurdetteLamar]

@kou, here's another PR. This completes all the methods in CSV::Table except #dig. I have to wait on a change from JeremyEvans for that.

[kou]

Thanks!