Last updated May 21, 2011.
This Drupal.org online documentation has style guides for documentation, as well as guidelines for how to file issue reports, how and when to comment and not to comment on Drupal documentation, etc.
These guides have been moved to the Drupal.org Style Guide
Additional guidelines found elsewhere on Drupal.org:
- Drupal UI Guidelines
- Doxygen formatting conventions (API code documentation)
- Module documentation guidelines
- Help text guidelines (hook_help() within modules)