Align Digest docstring with https://www.pantsbuild.org/v2.0/docs/ru…

…les-api-file-system

src/python/pants/engine/fs.py

@@ -15,13 +15,10 @@
 
 @dataclass(frozen=True)
 class Digest:
-    """A Digest is a content-digest fingerprint, and a length of underlying content.
+    """A Digest is a lightweight reference to a set of files known about by the engine.
 
-    Typically, this is used as a lightweight reference to a set of files known about by the engine.
     You can use `await Get(Snapshot, Digest)` to set the file names referred to, or use
     `await Get(DigestContents, Digest)` to see the actual file content.
-
-    Occasionally, a Digest is a reference for other types of strings/bytes/content.
     """
 
     fingerprint: str