Incorrect Doxygen documentation on tr_* APIs

[eschaton]

A bunch of the Doxygen documentation for the tr_* APIs says that they return NULL for a TestResult that isn't a failure.

For example, this is how tr_tcname() is documented:

/**
 * Retrieve test case name in which a failure occurred, if applicable.
 *
 * @return If the test resulted in a failure, returns a string
 *          containing the name of the test suite where the failure
 *          occurred; otherwise returns NULL.
 *
 * @since 0.6.0
 */

This appears to be incorrect; a TestResult appears to always contain full information about the test to which it applies. A cleanup pass through this documentation is warranted to ensure it matches the behavior.