`entryfile` in Project.toml

I recently discovered that if you have an entryfile entry in your package’s Project.toml file, you can put the package’s top-level module somewhere other than the typical src/Package.jl. This Project.toml, for example, works for a file at the specified path:

name = "EntryfileTest"
uuid = "88708527-90b9-4996-ac61-aaaaaaaaaaaa"
version = "0.1.0"
authors = ["author <author@test.com>"]
entryfile = "src/other_src/EntryfileTest.jl"

When I looked for information on this feature, I couldn’t find any info on it in the Pkg.jl docs; in fact, the only “official” place I could find it even mentioned was buried halfway in the Code Loading section of the manual.

Why is this? It seems like a handy feature. Is this just not part of the public API?