I always like to mention my size and needs when replying to posts like this because what works well for one company will be hopelessly inadequate for another. I work for a very small company, and most of my clients don't have VLANs (in fact, most don't even have managed switches) so I don't need anything too fancy.
I used to use Word documents in the past (along with some photos on a network share), which worked well for my simple needs. These days I need access when I'm out and about so I've changed to Confluence (a wiki). When I add a new customer I just create a new space from a template and fill in the details. You can define common blocks of text (like contact details for companies) and include them in your pages, so you don't need to do a whole lot of copying and pasting when you make changes.
The table support in Confluence is pretty good (although you don't get anything fancy like formula's). Confluence lets you add attachments to pages so I use it as the central spot for other stuff too (quotes, project documentation and notes, photos, client documentation - like how to set up their iPhone with their Exchange account etc). I've set it up with WebDAV so I can map a drive letter to get to all of these files, and can access them from an app on my phone. It also versions everything (pages, and attachments) which is important.
A lot of the online diagramming tools (like Gliffy, LucidChart, Draw.io and Creately) integrate with Confluence. You just insert a diagram where you want it in your documentation and you're taken to the online tool. Hit 'Save' and your diagram is saved automatically to Confluence (along with an 'Edit' button so you can make changes again in the future without mucking around in an 'edit, save to desktop, upload new version' cycle).
There's a plugin that lets you define some text that only appears when you click a link to expose it. I haven't decided yet, but I may well use this to avoid the issue of people looking over my shoulder if I look up some documentation when at a client site.
It's not perfect. For example I create new documentation from a template, but if I change the template in the future I have to manually go and make the change to all existing documents. (Apparently there's a plugin that does this but I haven't tested it out yet). As far as I'm aware Confluence doesn't have integration with any RMM or discovery tools, so you'll have to grab the information you need yourself and manually type it in.
I wanted to go with very cheap tools and if you have fewer than 10 techs that need edit permissions then this fits the bill. If you host it yourself Confluence is $10 (once-off). Draw.io and Creately looked to have the best network diagram features and they are also $10 for perpetual licenses. Pretty good value for money!