rhi: Add missing docs for srb create function

Change-Id: I573d8b95dae7db39f968a8b55aa2adcb14a73fdc
Reviewed-by: Andy Nichols <andy.nichols@qt.io>
(cherry picked from commit 3183b7b79126551fe8528a6fbcf6e90830ee695a)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
(cherry picked from commit b70a755fa2807e2b6a808ded0dfde04ccd57394f)
(cherry picked from commit 8393c41451d508030b6ef62fb536e41a0939cfcc)

src/gui/rhi/qrhi.cpp

@@ -5448,6 +5448,20 @@ void QRhiImplementation::updateLayoutDesc(QRhiShaderResourceBindings *srb)
     }
 }
 
+/*!
+    \fn virtual bool QRhiShaderResourceBindings::create() = 0
+
+    Creates the corresponding resource binding set. Depending on the underlying
+    graphics API, this may involve creating native graphics resources, and
+    therefore it should not be assumed that this is a cheap operation.
+
+    If create() has been called before with no corresponding destroy(), then
+    destroy() is called implicitly first.
+
+    \return \c true when successful, \c false when failed.
+    Regardless of the return value, calling destroy() is always safe.
+ */
+
 /*!
     \fn void QRhiShaderResourceBindings::setBindings(std::initializer_list<QRhiShaderResourceBinding> list)
     Sets the \a list of bindings.