[llvm] Add comment about the structure of the CodeGen (#6150)

Issue: #5511 

### Brief Summary

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Ailing  <ailzhang@users.noreply.github.com>

taichi/codegen/codegen.h

@@ -12,6 +12,29 @@
 #endif
 namespace taichi::lang {
 
+/*
+ [Note] Codegen of LLVM-based backends
+ * KernelCodeGen is the base class of the codegen of all backends using LLVM.
+ * Function `compile_to_function` first compiles the IR of a kernel
+ * into a LLVM module using `compile_kernel_to_module`, and then constructs a
+ * function for runtime execution using `ModuleToFunctionConverter`.
+ *
+ * Function `compile_kernel_to_module` compiles the IR of a kernel into a LLVM
+ * module. A kernel is composed of several offloaded tasks. To compile a kernel,
+ * we first compile each task independently into an LLVM module using function
+ * `compile_task`. Then, we link the LLVM modules of the offloaded tasks,
+ * the runtime module and the struct modules of the SNode trees which are used
+ * in the kernel all together into a single LLVM module using
+ * `tlctx->link_compiled_tasks`. The LLVM module and the names of the entry
+ * functions of the offloaded tasks in the module are stored in the returned
+ * LLVMCompiledKernel.
+ *
+ * Function `compile_task` uses `TaskCodeGen` of the respective backend to
+ * compile the IR of a offloaded task to an LLVM module. It also generates some
+ * extra information for linking such as which SNode tree is used in the task.
+ * The LLVM module, the name of the entry function of the offloaded task in the
+ * module and the extra information are stored in the returned LLVMCompiledTask.
+ */
 class KernelCodeGen {
  protected:
   Program *prog;