[docs] explain use of Preferences.jl to override JLL (#1126)

* [docs] explain use of Preferences.jl to override JLL

* Update jll.md

[skip azp]

Author: odow

docs/src/jll.md

@@ -282,3 +282,35 @@ Some comments about how to write this file:
   you want to use to override the products of the JLL package must have the same
   tree structure as the artifact.  In our example we need to use `/usr` to
   override Fontconfig and `/usr/local` for Bzip2.
+
+### Overriding specific products
+
+Instead of overriding the entire artifact, you can override a particular product
+(library, executable, or file) within a JLL using
+[Preferences.jl](https://github.com/JuliaPackaging/Preferences.jl).
+
+!!! compat
+    This section requires Julia 1.6 or later.
+
+For example, to override our `libbz2` example:
+```julia
+using Preferences
+set_preferences!(
+    "LocalPreferences.toml",
+    "Bzip2_jll",
+    "libbzip2_path" => "/usr/local/lib/libbz2.so",
+)
+```
+Note that the product name is `libbzip2`, but we use `libbzip2_path`.
+
+!!! warning
+    There are two common cases where this will not work:
+    1. The JLL is part of the [Julia stdlib](https://github.com/JuliaLang/julia/tree/master/stdlib),
+       for example `Zlib_jll`
+    2. The JLL has not been compiled with [JLLWrappers.jl](https://github.com/JuliaPackaging/JLLWrappers.jl)
+       as a dependency. In this case, it means that the last build of the JLL
+       pre-dates the introduction of the JLLWrappers package and needs a fresh
+       build. Please open an issue on [Yggdrasil](https://github.com/JuliaPackaging/Yggdrasil/)
+       requesting a new build, or make a pull request to update the relevant
+       `build_tarballs.jl` script.
+