How do you document the functionality of an application?

Question

Documentation

Hello.
The question is, is there any tool for describing the functionality of the future application? At the moment we are doing everything just in a text document, but I had an idea that perhaps there is some specialized tool for this. I would like to be able to describe each function of the future application and be able to somehow structure it all, for example, write tags for each function, such as "available only for pro accounts", put down some properties, for example, such as a menu item, under which this function should be.
Who uses what for similar tasks?
Thanks in advance.

Answer 1

We have a whole semester for the subject "Object-oriented analysis and design" was allotted. We built various UML diagrams in Modelio. The software often lagged .. In general, I passed the course and forgot.

Answer 2

1. How big is the software in terms of functionality now and is supposed to be?
2. What type of information do you want to post there? High-level requirements, technical requirements, architecture, user documentation, or both?