Additional guidance for Doxygen comments should help improve the quality of generated documentation.

Changes proposed in this pull request

The Doxygen C++ style instructions are updated as follows:

C++

Doxygen Comments

• All classes, member variables, and methods should use Doxygen-style comments.
• omments should provide brief and/or detailed descriptions. For example, comment blocks starting with /** or //! use the autobrief feature (comments are split into brief and detailed descriptions at the first dot '.'). For short comments, the C++ style //! is preferred; do not use /// or /*! comment styles in new code.
• Doxygen commands should use the @ prefix instead of \ in order to better differentiate from LaTeX input.
• Whenever appropriate, classes and functions should be added to Doxygen groupings using the @ingroup command. Alternatively, entire code sections can be added using the @addtogroup command, where grouped classes and functions are bracketed by @{ and @}.
• If applicable, new features should reference literature using the @cite command, with BibTeX-style entries added to cantera.bib. Equations can be added using LaTeX input bracketed by @f[ and @f]. In-line math expressions are enclosed by a pair of @f$ directives, for example @f$ \sin(x) @f$.
• Indicate the version added for new functions and classes with an annotation like @since New in %Cantera X.Y where X.Y is the next Cantera version. This notation should also be used to indicate significant changes in behavior.

Style Guide

• Avoid defining non-trivial functions in header files
  [...]

Also, add instructions on where std:: can be omitted.

If applicable, fill in the issue number this pull request is fixing

Addresses:

• Use Doxygen effectively enhancements#179

Checklist

• The pull request includes a clear description of this code change
• Commit messages have short titles and reference relevant issues
• Build passes (scons build & scons test) and unit tests address code coverage
• Style & formatting of contributed code follows contributing guidelines
• The pull request is ready for review