Document behavior of multi-field convert-with-row

[chrullrich]

Fixes #531.

This PR has the objective of... setting in stone the fact that convert() works the way it does by design, not accident.

Changes

1. Improved the docs about... petl.transform.conversions.convert() when used with multiple conversions in its whole-row mode.

Adds a doctest. doctests pass; I cannot get the whole test setup working at all, let alone actually test anything.

Checklist

Checklist for for pull requests including new code and/or changes to existing code...

• Code
  • Includes unit tests
  • New functions have docstrings with examples that can be run with doctest
  • New functions are included in API docs
  • Docstrings include notes for any changes to API or behaviour
  • All changes documented in docs/changes.rst
• Testing
  • (Optional) Tested local against remote servers
  • Travis CI passes (unit tests run under Linux)
  • AppVeyor CI passes (unit tests run under Windows)
  • Unit test coverage has not decreased (see Coveralls)
• Changes
  • (Optional) Just a proof of concept
  • (Optional) Work in progress
  • Ready to review
  • Ready to merge

[coveralls]

Coverage decreased (-0.003%) to 91.94% when pulling f042b28 on chrullrich:issue-531 into b8068f8 on petl-developers:master.