You are joining the development team of an existing Android app and are handed a technical document about it.
What information would you like it to have?
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Probably not the answer you are looking for, but I don't want to have to read a technical document to understand an application.
In my opinion, the most important piece of documentation for a project is the unit tests! Whenever I come across some code I don't understand the importance of, I check the test cases. Clear and comprehensive tests can speak volumes about what the code is used for, along with all the other great benefits.
One problem with technical documents, is that they become obsolete very quickly. As features get added, and source code is refactored, maintaining documentation becomes a big hassle.
Technical documents should be geared towards your clients, users, or business partners, not your developers.