To me, a coding style guide is a writers guide, similar to what you'd see used with publications. It is part of how a design is communicated (specs, requirements and, architecture) an implementation is communicated. Communication of its implementation (coding style, comments, review) its testing, what can be expected of it (the code).
Although there may be different coding style guides for different projects, each guide typically covers such things as:
Some of the goals are to allow code analysis or review by engineers on other software teams. Another is to help allow automated analysis or testing. But another is the role of a style guide in process (_what_ are you trying to communicate with process?), a topic I may ponder in the future.
Other Coding Style Guide It can help to study other style guides (or refer to them). Below are a couple. Multics Coding Standards
I can't recall a Unix style guide per se, but I believe each org (e.g. HP, Bell Labs) had one. You may wish to contrast this with GNU's guide and the 'non' guide
Michael Barr sells a coding standard, but it seems more like a power point presentation for his consulting or a starting outline of what some of a style guide would contain, rather than full, representative style guide. But it is also a good read.