Merge pull request #25591 from LaurentBerger:I25589

Remove dnn::layer::allocate in doc #25591

### Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

- [x] I agree to contribute to the project under Apache 2 License.
- [x] To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
- [x] The PR is proposed to the proper branch
- [x] There is a reference to the original bug report and related work #25589 
- [ ] There is accuracy test, performance test and test data in opencv_extra repository, if applicable
      Patch to opencv_extra has the same branch name.
- [ ] The feature is well documented and sample code can be built with the project CMake

modules/dnn/include/opencv2/dnn/dnn.hpp

@@ -214,7 +214,7 @@ CV__DNN_INLINE_NS_BEGIN
 
     /** @brief This interface class allows to build new Layers - are building blocks of networks.
      *
-     * Each class, derived from Layer, must implement allocate() methods to declare own outputs and forward() to compute outputs.
+     * Each class, derived from Layer, must implement forward() method to compute outputs.
      * Also before using the new layer into networks you must register your layer by using one of @ref dnnLayerFactory "LayerFactory" macros.
      */
     class CV_EXPORTS_W Layer : public Algorithm