doc: fix regression that deleted all methods from the reference manual

Regressed in commit bfb1222.

This needs to iterate over all methods, process them, and add them to a
list. Instead, it deleted all methods, processed all remaining methods,
and appended them to the in-use iterator.

Use a second list, instead.

Fixes mesonbuild#9922

Author: eli-schwartz

docs/refman/loaderyaml.py

@@ -274,12 +274,12 @@ def as_methods(mlist: T.List[Function]) -> T.List[Method]:
         methods = raw.pop('methods', [])
         obj = Object(methods=[], obj_type=obj_type, **raw)
 
-        methods = []
+        newmethods = []
         for x in methods:
             if not self.strict:
                 x = {**self.template.s_function, **x}
-            methods += [self._process_function_base(x, obj)]
-        obj.methods = as_methods(methods)
+            newmethods += [self._process_function_base(x, obj)]
+        obj.methods = as_methods(newmethods)
         return obj
 
     def _load_module(self, path: Path) -> T.List[Object]: