Documentation for julia hooks

See pre-commit PR: pre-commit/pre-commit#3348

Author: fredrikekre

sections/new-hooks.md

@@ -332,15 +332,22 @@ installed.  It has been tested on linux, macOS, and windows.
 
 ### julia
 
-_new in 4.x.0_
-
-The hook repository must contain a `Project.toml` file and optionally a `Manifest.toml`
-file. Hooks will run in this environment.
+_new in 4.1.0_
 
 For configuring julia hooks, your [`entry`](#hooks-entry) should be a path to a julia source
 file relative to the hook repository (optionally with arguments).
 
-For example:
+Hooks run in an isolated package environment defined by a `Project.toml` file (optionally
+with a `Manifest.toml` file) in the hook repository. If no `Project.toml` file is found the
+hook is run in an empty environment.
+
+Julia hooks support [`additional_dependencies`](#config-additional_dependencies) which can
+be used to augment, or override, the existing environment in the hooks repository. This also
+means that julia can be used as a `repo: local` hook. `additional_dependencies` are passed
+to `pkg> add` and should be specified using
+[Pkg REPL mode syntax](https://pkgdocs.julialang.org/v1/repl/#repl-add).
+
+Examples:
 
 ```yaml
 - id: foo-without-args
@@ -351,14 +358,15 @@ For example:
   name: ...
   language: julia
   entry: bin/bar.jl --arg1 --arg2
+- id: baz-with-extra-deps
+  name: ...
+  language: julia
+  entry: bin/baz.jl
+  additional_dependencies:
+   - 'ExtraDepA@1'
+   - '[email protected]'
 ```
 
-This language supports `additional_dependencies` so it can be used as a `repo: local` hook.
-`additional_dependencies` are passed to `pkg> add` and should be specified using
-[Pkg REPL mode syntax](https://pkgdocs.julialang.org/v1/repl/#repl-add).
-Note that `additional_dependencies` can be used to override package versions in the
-`Project.toml` file of the hook repository.
-
 __Support:__ julia hooks are known to work on any system which has `julia` installed.
 
 ### lua