Usually default.cfg and intro.txt are kept in sync with the same text

Where and what is default.cfg?

default.cfg is as its name suggest the default configuration for the game. When not overriden by user.cfg, all config values are read from there.
You can find it here:

ps/trunk/binaries/data/config/default.cfg

Thanks, that explains why I couldn't find it in the public folder!

Here an example of how a default.cfg / intro.txt change is properly applied rP21173, rP22650.
Here an example where they are mostly similar but differ rP21339 that I wanted to fix in D2273.

For example this patch correctly changed

Alt + C: Toggle attack range visualizations of selected defensive structures

to

Alt + C – Toggle attack range visualizations of selected units and structures

But in default.cfg we still have:

session.toggleattackrange = "Alt+C" ; Toggle display of attack range overlays of selected defensive structures

In fact, when changing hotkeys, there have to be three places kept in sync, the third one is this:
https://trac.wildfiregames.com/wiki/HotKeys

The more the files go out of sync, the harder it will be to keep them in sync.
If the files contain actually differing descriptions what the hotkey does (for example one says that the hotkey opens the page, and the other says it opens or closes the page) then it will become even hard or impossible to figure out what the hotkey was actually meant to do.
(I suppose if the changes in this diff are worthwhile for reasons, they are also worthwhile in the other places for the same reasons)