mruby-sprintf: add comprehensive call-seq documentation for String#% method

- %: string formatting operator that uses the string as a format specification
  and applies it to the given argument(s), supports both single arguments and
  arrays for multiple substitutions, delegates to sprintf for actual formatting

The method now has comprehensive call-seq documentation with practical
examples demonstrating various sprintf formatting patterns including:

- Zero-padded integers: "%05d" % 123
- Multiple substitutions with arrays: "%-5s: %016x" % [name, id]
- Hash-based named substitutions: "foo = %{foo}" % { :foo => 'bar' }
- Named format specifiers: "%{foo}f" % { :foo => 1 }

Co-authored-by: Atlassian Rovo Dev

mrbgems/mruby-sprintf/mrblib/string.rb

@@ -1,4 +1,19 @@
 class String
+  #
+  # call-seq:
+  #   str % arg   -> new_str
+  #   str % args  -> new_str
+  #
+  # Format - Uses str as a format specification, and returns the result
+  # of applying it to arg. If the format specification contains more than
+  # one substitution, then arg must be an Array or Hash containing the values
+  # to be substituted. See sprintf for details of the format string.
+  #
+  #   "%05d" % 123                              #=> "00123"
+  #   "%-5s: %016x" % [ "ID", self.object_id ]  #=> "ID   : 00002b054ec93168"
+  #   "foo = %{foo}" % { :foo => 'bar' }        #=> "foo = bar"
+  #   "%{foo}f" % { :foo => 1 }                 #=> "1f"
+  #
   def %(args)
     if args.is_a? Array
       sprintf(self, *args)