Add descriptive comments for MRB_API functions in src/proc.c

This commit adds Doxygen-style comments to the following MRB_API functions in src/proc.c:
- mrb_proc_new_cfunc
- mrb_proc_new_cfunc_with_env
- mrb_closure_new_cfunc
- mrb_proc_cfunc_env_get

The comments describe the purpose, parameters, and return value of each function.

src/proc.c

@@ -130,6 +130,14 @@ mrb_closure_new(mrb_state *mrb, const mrb_irep *irep)
   return p;
 }
 
+/*
+ * Creates a new Proc object from a C function.
+ *
+ * mrb: The mruby state.
+ * func: The C function to create the Proc from.
+ *
+ * Returns a pointer to the new Proc object.
+ */
 MRB_API struct RProc*
 mrb_proc_new_cfunc(mrb_state *mrb, mrb_func_t func)
 {
@@ -144,6 +152,17 @@ mrb_proc_new_cfunc(mrb_state *mrb, mrb_func_t func)
   return p;
 }
 
+/*
+ * Creates a new Proc object from a C function with a new environment.
+ *
+ * mrb: The mruby state.
+ * func: The C function to create the Proc from.
+ * argc: The number of arguments to be stored in the environment.
+ * argv: The array of mrb_value to be stored in the environment.
+ *       These values are copied into the environment.
+ *
+ * Returns a pointer to the new Proc object.
+ */
 MRB_API struct RProc*
 mrb_proc_new_cfunc_with_env(mrb_state *mrb, mrb_func_t func, mrb_int argc, const mrb_value *argv)
 {
@@ -172,12 +191,44 @@ mrb_proc_new_cfunc_with_env(mrb_state *mrb, mrb_func_t func, mrb_int argc, const
   return p;
 }
 
+/*
+ * Creates a new closure from a C function.
+ *
+ * This function creates a new Proc object that represents a C function
+ * along with an environment for a specified number of local variables.
+ * The local variables in the environment are initialized to nil.
+ *
+ * mrb: The mruby state.
+ * func: The C function to create the closure from.
+ * nlocals: The number of local variables to allocate space for in the
+ *          closure's environment.
+ *
+ * Returns a pointer to the new Proc object representing the closure.
+ */
 MRB_API struct RProc*
 mrb_closure_new_cfunc(mrb_state *mrb, mrb_func_t func, int nlocals)
 {
   return mrb_proc_new_cfunc_with_env(mrb, func, nlocals, NULL);
 }
 
+/*
+ * Retrieves a value from the environment of the currently executing C function Proc.
+ *
+ * This function is used within a C function that was wrapped into a Proc
+ * (e.g., using mrb_closure_new_cfunc or mrb_proc_new_cfunc_with_env)
+ * to access values stored in its associated environment.
+ *
+ * mrb: The mruby state.
+ * idx: The index of the value to retrieve from the C function's environment.
+ *      The index must be non-negative and less than the number of
+ *      environment entries (argc passed during creation).
+ *
+ * Returns the mrb_value stored at the specified index in the environment.
+ *
+ * Raises E_TYPE_ERROR if the currently executing Proc is not a C function
+ * or if it does not have an associated environment.
+ * Raises E_INDEX_ERROR if the provided index is out of bounds.
+ */
 MRB_API mrb_value
 mrb_proc_cfunc_env_get(mrb_state *mrb, mrb_int idx)
 {