Modify registration recommendations

[wthrowe]

This change makes derived classes using FakeVirtual stuff usable as
concrete classes without specifying the registrars parameter. All
current uses of the registration framework fall into this category.

Proposed changes

Types of changes:

• Bugfix
• New feature

Component:

• Code
• Documentation
• Build system
• Continuous integration

Code review checklist

• The PR passes all checks, including unit tests, clang-tidy and IWYU.
  For instructions on how to perform the CI checks locally refer to the Dev
  guide on the Travis CI.
• The code is documented and the documentation renders correctly. Run
  make doc to generate the documentation locally into BUILD_DIR/docs/html.
  Then open index.html.
• The code follows the stylistic and code quality guidelines listed in the
  code review guide.

Further comments

[kidder]

Travis failures are from #1031