[Doc] Expand Multimodal API Reference (#11852)

Signed-off-by: DarkLight1337 <tlleungac@connect.ust.hk>

docs/source/api/multimodal/inputs.md

@@ -0,0 +1,49 @@
+# Input Definitions
+
+## User-facing inputs
+
+```{eval-rst}
+.. autodata:: vllm.multimodal.MultiModalDataDict
+```
+
+## Internal data structures
+
+```{eval-rst}
+.. autoclass:: vllm.multimodal.inputs.PlaceholderRange
+    :members:
+    :show-inheritance:
+```
+
+```{eval-rst}
+.. autodata:: vllm.multimodal.inputs.NestedTensors
+```
+
+```{eval-rst}
+.. autoclass:: vllm.multimodal.inputs.MultiModalFieldElem
+    :members:
+    :show-inheritance:
+```
+
+```{eval-rst}
+.. autoclass:: vllm.multimodal.inputs.MultiModalFieldConfig
+    :members:
+    :show-inheritance:
+```
+
+```{eval-rst}
+.. autoclass:: vllm.multimodal.inputs.MultiModalKwargsItem
+    :members:
+    :show-inheritance:
+```
+
+```{eval-rst}
+.. autoclass:: vllm.multimodal.inputs.MultiModalKwargs
+    :members:
+    :show-inheritance:
+```
+
+```{eval-rst}
+.. autoclass:: vllm.multimodal.inputs.MultiModalInputsV2
+    :members:
+    :show-inheritance:
+```