Add docstrings for ArtifactLoader values (#1179)

**Pull Request Checklist**
- [x] Fixes doc string for ArtifactLoaders
- [ ] ~Tests added~
- [x] Documentation/examples added
- [x] [Good commit messages](https://cbea.ms/git-commit/) and/or PR
title

**Description of PR**
When trying to use `ArtifactLoaders` I found it hard to remember what
`json` and `file` actually do, and what type the variable type should
be. This PR adds docstrings which help when you're writing the
annotation.

Signed-off-by: Elliot Gunton <[email protected]>

src/hera/workflows/artifact.py

@@ -39,7 +39,10 @@ class ArtifactLoader(Enum):
     """Enum for artifact loader options."""
 
     json = "json"
+    """Deserialize the JSON-string Artifact file to a Python object (the target variable can be any JSON-compatible type)."""
+
     file = "file"
+    """Read the contents of the Artifact file directly as a string (the target variable must be a `str` type)."""
 
 
 class Artifact(BaseModel):