Is there a software documentation standard where the developers need to create only one document for their modules and it can be used as a functional doc as well as for testing the software.
Edit: I am not looking for API documentation. A document which will include specs which can be used as a functional and testing doc.
I don't quite understand the question, and it's certainly language dependent, but look into Javadocs if you're looking for powerful, autogenerated documentation that moves with your source code. This exists for Java (obviously) and for PHP, as far as I know, but it would be perfectly possible for someone to create such a tool for most langauges if they don't exist already. Javadocs let you basically put comments in your code and then they are transformed into a set of webpages to easily browse and navigate.
I don;t think there is any such "standard" but it does make sense to have a single document that contains the functional specs that can also be used as acceptance test cases.
I assume you are talking about API documentation, aimed at the consumers of a library, rather than developer documentation or end-user application documentation.
I don't think there can be such as standard. The requirements of testing are different to those of documentation.
Typically as a user I am concerned with the common case.
As a tester I am concerned with edge cases.
However, developer documentation does have overlap with testing, for example ASSERTIONS can be read by the developer about properties of the program that are not expressed in the code. They also are used by the tester to check the developer hasn't broken these properties.