[Common FE] Document get_input_by_reference better (openvinotoolkit#26165)

**Details:** Document get_input_by_reference better

**Ticket:** TBD

Signed-off-by: Kazantsev, Roman <roman.kazantsev@intel.com>

Author: rkazants

src/frontends/common/include/openvino/frontend/node_context.hpp

@@ -48,7 +48,12 @@ class FRONTEND_API NodeContext {
         FRONT_END_NOT_IMPLEMENTED(get_input);
     }
 
-    /// \brief Returns the input by reference. The reference value can be changed by consuming operation
+    /// \brief Returns output of Variable node (or Variable value).
+    /// Variable is a special node that stores a value represented with a sub-graph.
+    /// Variable has a concrete value at each conversion step.
+    /// The current (consuming) operation node can change its value
+    /// so consumers of this Variable will have a new value at next conversion steps.
+    /// See ov::frontend::Variable class for more details.
     virtual Output<Node> get_input_by_reference(int idx) const {
         FRONT_END_NOT_IMPLEMENTED(get_input_by_reference);
     }