limitations.md: document nested def scope in singleton methods

mruby places `def` written inside `def self.foo` on the receiver's
singleton class (making it a class method of the enclosing class).
CRuby places it as an instance method of the lexical enclosing
class. This is a long-standing divergence that we've chosen to
document rather than change.

close #1536

Co-authored-by: Claude <noreply@anthropic.com>

doc/limitations.md

@@ -335,3 +335,38 @@ the splat operator (`*obj`).
 This is a deliberate trade-off: implicit conversion forces every
 coercion site to go through method dispatch and can silently mask
 type-mismatch bugs.
+
+## Nested `def` in Singleton-Method Context
+
+`def` written inside a singleton method (`def self.foo`) is placed
+on a different class in mruby than in CRuby. CRuby registers the
+inner method as an instance method of the lexical enclosing class.
+mruby registers it as a method of the enclosing receiver's
+singleton class, which makes it visible as a class method of the
+enclosing class.
+
+```ruby
+class SomeClass
+  def self.class_method
+    def nested; 'nested!'; end
+  end
+end
+SomeClass.class_method
+```
+
+#### CRuby
+
+```
+SomeClass.nested        # NoMethodError
+SomeClass.new.nested    # => "nested!"   (instance method)
+```
+
+#### mruby
+
+```
+SomeClass.nested        # => "nested!"   (class method)
+SomeClass.new.nested    # NoMethodError
+```
+
+Writing nested `def` like this is unusual; this difference rarely
+surfaces in practical code.