docs: Clarify only DMA-BUFs with a single modifier are supported

The gs_texture_create_from_dmabuf function is a wrapper for
eglCreateImageKHR, which can set a different modifier for each plane.
The documentation should advice against that, since this should be
rejected by the driver.

While DMA-BUFs started to have one modifier per plane, this decision was
rolled back to having one modifier for the whole buffer / all planes.
While some apis like EGL were designed with a modifier per plane,
drivers are expected to reject any request using different modifiers per
plane.

docs/sphinx/reference-libobs-graphics-graphics.rst

@@ -980,6 +980,9 @@ Texture Functions
    careful to always try and match the formats correctly, otherwise textures
    can fail to be created or rendered.
 
+   All modifiers passed in the modifiers array must be equal. Passing different
+   modifiers for each plane is unsupported.
+
    :param width:        Width of the texture
    :param height:       Height of the texture
    :param drm_format:   DRM format of the DMA-BUF buffer