HowTo write correct Changes after my C++ project was updated?

Question

Hi folks!

Here what I have got:

1. Microsoft Visual Studio / Visual C++ IDE is used
2. Qt SDK version 4.6.2
3. Committing code changes with TortoiseSVN client
4. Project with more then 20K lines of code (mostly without comments) with UIs, resources...

Problem: after committing new portion of code/source changes, I'd like to write in comments correct data, what was changed, what was updated in GUI

Question: is there any documentations, tutors or manuals about this?

Thanks a lot.