Documentation writers for C and C++

[JaredWright]

PAL's Rust code generator supports writing file-level and function-level documentation strings that can be used to create friendly documentation pages (see pull-requrest #97). An equivalent functionality should be developed for both C and C++, generating something along the lines of doxygen-style comments.

This task consists of:

• Implement the declare_file_documentation and declare_item_documentation methods on the existing C/C++ comment writer
• Update the C and C++ generators to make usage of these new methods, wherever they make sense.
• Evaluate the need for the existing declare_comment interface method. Is this used by anything after these changes occur?