I would just like to ask if it is possible for ODK to have a note / comment button especially if have a survey and the enumerator wants to input some comments or notes for specific section (if necessary)
or "text" question type is the only option for this one?
Your help would be much appreciated as I am just new to ODK. 
Thank you!
Text as the questiontype
https://docs.getodk.org/form-question-types/#text-widgets
I hope that you mean this
There is no such button and as @Dominik1 you can use just a text question.
If you need it to appear only in some cases you can add a select_one question before like:
Would you like to add some comments? (yes/no) and then display that text question only if a user selected yes (using relevant https://xlsform.org/en/#relevant).
I see, thank you for your help.