docs: align the remaining Decoder and Encoder mentions with the options API

Assisted-by: GLM 5.3 Flash

docs/API.md

@@ -547,12 +547,12 @@ sequenceDiagram
     participant Caller
     participant Unmarshal as Unmarshal
     participant Parser as parser
-    participant Decoder as decoder
+    participant Decode as decode
     Caller->>Unmarshal: data, v
-    Unmarshal->>Parser: ParseContext(ctx, data)
-    Parser-->>Unmarshal: tree or *SyntaxError
-    Unmarshal->>Decoder: decode(tree, reflect value)
-    Decoder-->>Unmarshal: nil or wrapped field error
+    Unmarshal->>Parser: targeted parse straight into v
+    Parser-->>Unmarshal: nil, *SyntaxError, or fallback
+    Unmarshal->>Decode: tree rerun on fallback
+    Decode-->>Unmarshal: nil or wrapped field error
     Unmarshal-->>Caller: error
 ```
 
@@ -647,7 +647,7 @@ parsed as keys of the sub-table.
 
 ### Preserving declaration order
 
-`LayoutKindDeclaration` on an `Encoder` walks the entries in declaration order
+`Layout(LayoutKindDeclaration)` walks the entries in declaration order
 instead, emitting each header immediately before its content:
 
 ```go