It's become somewhat of a running joke that programmers don't like to write documentation and thus refer to the code that they've written as self-documenting code. The reality is that without clear documentation of the design requirements, architecture overview, design plans, and with the API documentation, you risk the future of the project and both one's fellow developers and the end-users who rely on the software to function.

Following procedures and doing all the boring paperwork before you can start writing the first line of code may seem like a complete killjoy. Unfortunately, the reality is that, without this effort, this knowledge will remain locked in the heads of the project's developers, which complicates the integration of the firmware into the rest of an embedded project and makes future maintenance, especially if moved to a different team, a daunting prospect.

The simple fact is that no code is self-documenting, and even if it were, no hardware engineer is going to go through thousands of lines of code to figure out what kind of signal is being put out on a specific GPIO pin when a particular input condition for the MCU occurs.