add comments + precompile statement

src/TZJData.jl

@@ -4,6 +4,11 @@ using Artifacts
 
 artifact_dir() = artifact"tzjdata"
 Base.@deprecate_binding ARTIFACT_DIR artifact_dir() false
+# Why not simply use `const ARTIFACT_DIR = artifact"tzjdata"`?
+# The problem is that `ARTIFACT_DIR` would be set to the artifact path at the time of precompilation.
+# If the Julia depot is afterwards relocated,
+# ARTIFACT_DIR will continue to point to the old no-longer-valid location.
+# This occurs for example if TimeZones.jl is included inside a system image.
 
 const TZDATA_VERSION = let
     artifact_dict = Artifacts.parse_toml(joinpath(@__DIR__, "..", "Artifacts.toml"))
@@ -12,4 +17,10 @@ const TZDATA_VERSION = let
     m !== nothing ? m[:version] : error("Unable to determine tzdata version")
 end
 
+precompile(artifact_dir, ())
+# `@deprecate_binding ARTIFACT_DIR artifact_dir()`
+# will have already precompiled `artifact_dir()`
+# but once the deprecated binding is removed
+# an explicit precompile statement will be necessary.
+
 end