Good sysadmin teams "think before they do." On a larger team it is important to communicate what you are about to do, or what you have done.
A design doc is a standardized format for proposing new things or describing current things. It should be short, 1-2 pages, but can be very long when the need arises.
Create a template and use it all over the place. The section headings might include: Overview, Goals, Non-Goals, Background, Proposed Solution, Alternatives Considered, Security, Disaster Recovery, Cost.
This format can be used to write a 20-page plan for how to restructure your network when you want to get buy-in from many people. It can be a 5-page document of how a prototype was built so everyone can see your results and give feedback before you build the real thing. It can be used for a half-page memo that explains the names you plan on using for a new directory tree on the file server (in which case, you probably don't need most of the headings). Use it to document a system after it was built for use as a reference by others on your team. Heck, use it to describe the team cookout you are planning.
The point is that your team has a mechanism for thinking before doing, a way to communicate plans beyond talking in hallways and chat-room, and a system that leaves behind artifacts that others can use to understand how or why something was done.
This format works when seeking feedback whether you want serious critiques or just "warn me if this will conflict with something you are about to do."
Your design doc format might have more headings or fewer, some may be optional, others may be required. Be flexible. A small project should only require a few headings. A huge project should require more of the headings. Be flexible. Have a "short form" and "long form". Having a standard template that people can use as a starting point avoids the "blank page syndrome".