Internal documentation is written by developers in or around the source code. You can cut your documentation time if you build your User Documentation at the same time you write the Internal or System Documentation.
But no one believes it or wants to hear it.
Nonetheless, a lot of information can be shared between the two. Even if you don’t want to or it is inappropriate to share information between the documents, you can benefit from topics in one document that will prompt you to include additional documentation in the other.
In cases where development is managed by an agile method, you are probably writing stories. In a lot of cases these reflect user cases. These story types can be used as a basis for user documentation, unless development went down the wrong track. In cases where testing involves writing scenarios… A lot of test cases can be moved to documentation or at least serves as source material.
The need is then to bridge the gap between the development cycle and the documentation cycle.
It means people need to share.