In addition to the overall description of the technical
structure of your product, the implementation
document will contain:
Class diagrams, activity diagrams (for parallel threads), state transition diagrams, interaction diagrams, ER diagrams (see OO design notes)
Your goal is to convey to the reader your mental model of the system design and construction.
Keep in mind the goals of the audience when they need to use your manual... changes, modifications, additions, maintenance. They will need to understand what you did, and they will want to learn this ASAP.
Put in this document whatever information such a person would need (or want) were he given the task of maintaining your code.
Remember to ZOOM... go from abstract to detailed. Allow the reader to enter the document at any of several levels. Give as much or as little detail as the reader wants at each level.
Remember you need an on-line version of this... a summary, containing your specs and diagrams at least.
One nice idea is to connect portions of the technical manual (and/or the user manual) to portions of the system diagrams.