Improve the readback docs

src/rhi/qrhi.cpp

@@ -3637,8 +3637,7 @@ void QRhiResourceUpdateBatch::copyTexture(QRhiTexture *dst, QRhiTexture *src, co
       QRhiReadbackResult *rbResult = new QRhiReadbackResult;
       rbResult->completed = [rbResult] {
           {
-              QImage::Format fmt = rbResult->format == QRhiTexture::BGRA8 ? QImage::Format_ARGB32_Premultiplied
-                                                                          : QImage::Format_RGBA8888_Premultiplied;
+              const QImage::Format fmt = QImage::Format_RGBA8888_Premultiplied; // fits QRhiTexture::RGBA8
               const uchar *p = reinterpret_cast<const uchar *>(rbResult->data.constData());
               QImage image(p, rbResult->pixelSize.width(), rbResult->pixelSize.height(), fmt);
               image.save("result.png");
@@ -3655,6 +3654,16 @@ void QRhiResourceUpdateBatch::copyTexture(QRhiTexture *dst, QRhiTexture *src, co
    \note The texture must be created with QRhiTexture::UsedAsTransferSource.
 
    \note Multisample textures cannot be read back.
+
+   \note The readback returns raw byte data, in order to allow the applications
+   to interpret it in any way they see fit. Be aware of the blending settings
+   of rendering code: if the blending is set up to rely on premultiplied alpha,
+   the results of the readback must also be interpreted as Premultiplied.
+
+   \note When interpreting the resulting raw data, be aware that the readback
+   happens with a byte ordered format. A \l{QRhiTexture::RGBA8}{RGBA8} texture
+   maps therefore to byte ordered QImage formats, such as,
+   QImage::Format_RGBA8888.
  */
 void QRhiResourceUpdateBatch::readBackTexture(const QRhiReadbackDescription &rb, QRhiReadbackResult *result)
 {

src/rhi/qrhigles2.cpp

@@ -1579,6 +1579,7 @@ void QRhiGles2::executeCommandBuffer(QRhiCommandBuffer *cb)
                 // readPixels handles multisample resolving implicitly
             }
             result->data.resize(result->pixelSize.width() * result->pixelSize.height() * 4);
+            // With GLES (2.0?) GL_RGBA is the only mandated readback format, so stick with it.
             f->glReadPixels(0, 0, result->pixelSize.width(), result->pixelSize.height(),
                             GL_RGBA, GL_UNSIGNED_BYTE,
                             result->data.data());