documenting a web app [duplicate]

My company is trying to improve the UX in our system. It's web based app, full with features.

The system development started 8 years ago, but it was never documented, and our programmers/support team can't remember every small thing about the features, that causes our clients who need support to wait while we dig into the source code to find things.

We want to document it, first for ourselves but later to publish some parts of it to our clients.

Since it's a big app, going through everything without a system/app could take forever and we probably won't be able to cover everything.

I was wondering if anyone knows about such system/app?

I need it to be easy to use for out support/sales teams and for the users but also efficient for the documenting process.