doc: clarify build.zig.zon urls should point to immutable data

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
* rcmagic/ZigFightingGame@4b64353

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

Not-Nik/raylib-zig#130

This updates the docs to clarify that urls should reference immutable
data.

doc/build.zig.zon.md

@@ -47,7 +47,8 @@ String.
 
 When updating this field to a new URL, be sure to delete the corresponding
 `hash`, otherwise you are communicating that you expect to find the old hash at
-the new URL.
+the new URL. The URL should always result in the same content, avoid URL's that
+reference content that can change such as git branch names.
 
 #### `hash`