docs: correct cache() docstring exception types to match the code

the docstring listed an unsupported content type under DPYCacheError, but _normalize/
_prevalidate raise ValueError for it (DPYCacheError is only the duplicate-filename and
missing-path cases). a caller following the docstring with except DPYCacheError would let
the ValueError escape. group the raises correctly: ValueError = unnamed bytes / unsupported
type; DPYCacheError = duplicate filename / missing path / count mismatch. doc-only.

Signed-off-by: disqualifier <dev@disqualifier.me>

src/dpy_cache/dpy_cache.py

@@ -121,12 +121,12 @@ class DPYCache:
         content: "str | None" = None,
     ) -> CacheResult:
         """upload files to the storage channel (batching over 10/message) and return refs
-        keyed by original filename; raises ValueError on unnamed bytes in a list input,
-        DPYCacheError on a duplicate filename in a list input, an unsupported content type,
-        or a missing file path (all checked before any upload, so a locally-detectable bad
-        input in a >10-file batch costs zero requests and orphans nothing), or an
-        attachment-count mismatch after a send, and propagates discord's own
-        Forbidden/HTTPException unwrapped on a send failure"""
+        keyed by original filename; raises ValueError on unnamed bytes in a list input or an
+        unsupported content type, DPYCacheError on a duplicate filename in a list input or a
+        missing file path (all checked before any upload, so a locally-detectable bad input
+        in a >10-file batch costs zero requests and orphans nothing) or an attachment-count
+        mismatch after a send, and propagates discord's own Forbidden/HTTPException unwrapped
+        on a send failure"""
         items = self._normalize(files)
         self._prevalidate(items)
         merged: dict[str, FileRef] = {}