There's been some proliferation of dependency URLs that reference
mutable data such as links to git branches that can change.  This has
resulted in broken projects, i.e.

* https://github.com/RohanVashisht1234/raylib_rain_train/blob/9eef9de94c511f2eb4fe5db1d6abd574ee245c9b/build.zig.zon
* https://github.com/rcmagic/ZigFightingGame/commit/4b64353e9c69de0fa2eb87fa9c3a3da76a8a3e7b

There's also disagreement about whether it's fine for URL's to point to
git branches, i.e.

https://github.com/Not-Nik/raylib-zig/pull/130

This updates the docs to mention that zig won't be able to use URLs if
their content changes.