clarify signature

mypy/plugins/attrs.py

@@ -934,8 +934,6 @@ def evolve_function_sig_callback(ctx: mypy.plugin.FunctionSigContext) -> Callabl
         return ctx.default_signature
     inst_type_str = format_type_bare(inst_type)
 
-    # In practice, we're taking the initializer generated by _add_init and tweaking it
-    # so that (a) its arguments are kw-only & optional, and (b) its return type is the instance's.
     attrs_init_type = _get_attrs_init_type(inst_type)
     if not attrs_init_type:
         ctx.api.fail(
@@ -944,6 +942,10 @@ def evolve_function_sig_callback(ctx: mypy.plugin.FunctionSigContext) -> Callabl
         )
         return ctx.default_signature
 
+    # AttrClass.__init__ has the following signature (or similar, if having kw-only & defaults):
+    #   def __init__(self, attr1: Type1, attr2: Type2) -> None:
+    # We want to generate a signature for evolve that looks like this:
+    #   def evolve(inst: AttrClass, *, attr1: Type1 = ..., attr2: Type2 = ...) -> AttrClass:
     return attrs_init_type.copy_modified(
         arg_names=["inst"] + attrs_init_type.arg_names[1:],
         arg_kinds=[ARG_POS] + [ARG_NAMED_OPT for _ in attrs_init_type.arg_kinds[1:]],