2544: explain the 'author missing' gemspec warning; change engine -> extension where appropriate

Changes to the Getting Started/ 3 - Generate an Extension... guide:

1. changed "engine" to "extension" where appropriate
2. updated the reference to the testing guide to use the correct guide title
3. Put in some explanation about the commands to run after generating the extension.  This will be obvious to anyone with some rails experience, but will provide some context and explanation to newbies.
4. Issue #2544 - put in an explanation of the bundler/RubyGem warning about missing author from the .gemspec file.  I'm not sure if there's agreement about what should/shouldn't be put in the guide for this issue.  The wording I put in starts with "When you run bundle install, you may see a message that says..."