How to find the help on the special keywords such as &optional in emacs lisp

Some of the arguments in the emacs lisp开发者_JAVA技巧 function contains the keyword &optional where to find the documentation of those keywords. I tried searching using describe-function and describe-variable without sucess.

I found the reference to &optional in the elisp manual of my Emacs24 (dev version).

Look in info C-h i m elisp RET m functions RET go to the second entry Lambda Expressions then Argument List.

&optionalis described in the third paragraph.

And it can be found online here.

The "&-keywords" are &optional (this is an argument that is either provided or defaults to nil) and &rest (this must be the last argument, its value is a list of all 'remaining' arguments to the function, once all required and optional arguments have been 'consumed').

A fuller explanation can be found in the emacs lisp manual, as per @seki's answer.