Edit symlinked files in Dropbox
The quick answer is that if you open Documents/thesis.tex after editing it in your Dropbox folder on another computer, you will see the changes. As @Mark said in a comment, a symlink is simply a pointer to another location on your computer. If that other location has its contents changed, those changes will be visible when using the symlink in Documents, just as they would if you examined the file directly in the Dropbox folder.