When making up a game design document, do you usually document every piece of code that goes into it, or do you just document info about the various libraries, art assets, items, characters, etc?

A GDD usually doesn't mention, or hardly mentions, code issues.
A separate Technology Design Document will cover game and tool architecture if required.

A GDD usually doesn't mention, or hardly mentions, code issues.
A separate Technology Design Document will cover game and tool architecture if required.

Ok thank you for clearing that up, that actually helps a lot. I really don't like to document my code and architecture (except for the design in general) until after I've written and reviewed it. Also, it seems like documenting code would clutter the document a lot and make it difficult for others to go through it.

Another question: What tools do you usually use when writing up a design document? I tend to use an office program like LibreOffice, but I'd like to find something else to do diagrams, charts, and tables in (although tables aren't that hard to do in LibreOffice, but if there's a better way to do them that would be nice too.)

When making up a game design document, do you usually document every piece of code that goes into it, or do you just document info about the various libraries, art assets, items, characters, etc?

None of the above. Planning for the technology goes into the TDD, not the GDD. Do you want to ask about Technical Design Documents rather than GDDs? If so, ask that in the For Beginners forum, not the Game Design forum.

Another question: What tools do you usually use when writing up a design document? I tend to use an office program like LibreOffice, but I'd like to find something else to do diagrams, charts, and tables in (although tables aren't that hard to do in LibreOffice, but if there's a better way to do them that would be nice too.)

The standard is Microsoft Word for the document itself. For diagrams you can use any paint utility (even MS Paint can make perfectly good diagrams), pasted into the doc as needed. For charts and tables, the standard is Microsoft Excel. The tables and charts can be pasted right into the doc.

Another question: What tools do you usually use when writing up a design document? I tend to use an office program like LibreOffice, but I'd like to find something else to do diagrams, charts, and tables in (although tables aren't that hard to do in LibreOffice, but if there's a better way to do them that would be nice too.)

I find Google docs very the same as MS Word but with sharing option which is very helpful if you have to communicate with your team remotely

+1 for Google Docs. You can always export to MS Word if you need to. And just to add a little more to all the great advice above, it's best to think of the GDD as a synopsis for the whole thing. I know it's tempting, but it doesn't need too much detail at this stage.

I build up the final design in a google site and use google docs text and spreadsheets for the work material when writing.