mruby-numeric-ext: add call-seq documentation for Integer#integer? method

Added missing call-seq documentation for Integer#integer? method in
mrblib/numeric_ext.rb to complete documentation coverage:

- integer?: returns true for Integer objects, completing the integer?
  method documentation across both Numeric and Integer classes with
  consistent formatting and practical examples

Co-authored-by: Atlassian Rovo Dev

mrbgems/mruby-numeric-ext/mrblib/numeric_ext.rb

@@ -110,6 +110,15 @@ class Integer
     -div(-other)
   end
 
+  ##
+  #  call-seq:
+  #    int.integer?  ->  true
+  #
+  #  Returns true since this is an Integer.
+  #
+  #  1.integer?     #=> true
+  #  42.integer?    #=> true
+  #
   def integer?
     true
   end