Clearer documentation on making suggestions for improvements to documentation

[hturner]

Is your feature request related to a problem? Please describe.
Currently it is not clear from the guide how to contribute suggestions for improvements to R's documentation.

Describe the solution you'd like

• In What is a Bug? we should say "If documentation is incorrect or unclear, this is a bug". Review the rest of this chapter to makes sure it is clear how to report a bug in the documentation
• In the documentation chapter we should make clear that people can contribute by making suggestions (refer back to bug chapter) and not just addressing existing issues.

Describe alternatives you've considered
We could limit the documentation of this issue to the Documentation chapter - probably this is where people will look first. In which case, we would still refer back to the Bug Tracking chapter for general information on Bugzilla/Bug tracking, but add clear sections on reporting bugs, suggesting improvements and contributing to the documentation. Then would could link people straight to the relevant section for help.

Additional context
N/A