Generally, good behaved applications have an entry in their man page that spells out these details for you, so you don't have to work out anything.
But since the use case is personal dotfiles, I imagine the user isn't going to forget that they set this up.
No, it doesn't. The point of modetc is precisely keep both myself and the programs happy: the files are actually stored where I like to keep them, but they can be accessed as if they were stored where the developer intended.
Ok, now you might have a dozen files which could contain the information, where the location of each file can be modified by environment variables. It's tolerable if you are working on something you change weekly, but a practical problem if you do it yearly or it's entirely new.
Of course, this doesn't help at all when software either doesn't have manpages, or doesn't include the list of files in the manpage. Just nitpicking your bash example.
When I'm searching for configuration file location, I do use '/FILES' or PgUp from the bottom of the manpage, so the length of the manpages is irrelevant.