There are many things to document: designs, changes, APIs, policies and procedures. Each document should focus on a specific audience. Sending a change notification to your own group could be terse: "I rebooted system3 because <reason>." Documenting an important procedure that your customers will be invoking should be well written and probably not terse.
Good documentation takes effort and thought. Documentation should be written for a known audience. Choose your audience before writing a document. Express your intended audience before you begin your document. If you're revising a document, make sure the intended audience will still benefit after your revisions.
Documentation is about content, audience and findability. If the content is wrong or out of date, then your documentation is hurting the situation. If you don't know your audience, you can't best help the people who most need your information. If your document can't be found, no one will know it exists.
The content of your document is very important. The assumptions of prior knowledge in your content must be molded around your chosen audience. For instance, don't use an acronym without defining it unless you are certain your audience will know what it is or how to find out what it is. Content isn't necessarily always text. Good documents include diagrams or links to other resources, where possible.
The medium (wiki, email, paper, etc) of your documentation should reflect the needs of that documentation. Don't document long-term information over email. A printed new-hire todo list is good to have on said new-hire's desk on the first day. A network design should be published (perhaps on your wiki) with details including the problem, the solution, and benefits.
If the medium is electronic, you need to consider findability. Findability means that any person needing a piece of information can find it. Findability is difficult to provide without good search facilities and/or a easily browsable structure. Books have indeces to enhance findability, and your documentation should, too. These days, a wiki is a reasonable choice for containing your documents as they help you provide search and structure which improves findability. If you make roll out a major change, update your documentation and send an email to your audience (interested parties) indicating the change and where to learn about it.
You probably have a whole bucket of things that merit documentation. Prioritize these by what will gain you the most first. If you get multiple interrupts a week from customers making a frequently asked question, documenting it and respond with "Look on the wiki for <foo>" is a good way keep an interrupt short. Helping customers help themselves helps you. Of course, by "customer" I mean the people you are, as a sysadmin, supporting. Even if they're other employees, they're still customers.
Documentation, like code, needs to be tested. Testing documentation means having someone from your intended audience read it and report their level of understanding. If they didn't understand the information you were expressing, then you need to revise and re-test. If a document is intended for your customers, don't have a fellow team member review it.
Further, documentation being like code, suffers bit rot if neglected. Unmaintained documentation means people reading it will be misinformed. Don't ignore your existing documentation. It's worth more to you updated than neglected.
Knowing that documentation is important means that you should prioritize the act of improving and creating documentation among your other duties and tasks. Such things take time and effort, so be sure to consider documentation when budgeting your own time on work.
Lastly, it's worth pointing out that some documentation can lead to automation. For example, a well-documented alerts (or failure scenario) playbook often looks like a flowchart detailing operations to perform to debug and fix a problem. This kind of detail often lends itself to being transformed into a script. Once you have the script, you could have your alert system run the new script instead of paging you, or even just use the script to automate collection of some diagnostic information to help you more quickly debug a problem.