IEEE Software Engineering standards Collection has some good stuff on requirements documentation. Also outlines Input and output documentation from each phase of a product life cycle.
You probably need to be a little more specific about what in the automation project you want to document. For example, the process via P&IDs, the software (batch or continuous; PLC/PC/DCS),loops and wiring, pneumatic instruments; operator interface, etc. ISA's (www.isa.org) focus is instrumentation, controls and automation has a number of standards dealing with these and other topics. The difficulty you may have is that there is no singular standard of which I am aware that ties all of these topics together for an "automation project". You will also find that the "standards" of documentation are more conventions and vary from industry to industry.