I suppose this is as much a standard as anything.
The Open Group Base Specifications Issue 7
IEEE Std 1003.1, 2013 Edition
Copyright © 2001-2013 The IEEE and The Open Group
Ch. 12 – Utility Conventions
Although it doesn’t seem to mention many things I have commonly seen over the years used to denote various meanings:
- square brackets
[optional option] - angle brackets
<required argument> - curly braces
{default values} - parenthesis
(miscellaneous info)
Edit: I should add, that these are just conventions. The important thing is to pick a convention which is sensible, clearly state your convention, and stick to it consistently. Be flexible and create conventions which seem to be most frequently encountered on your target platform(s). They will be the easiest for users to adapt to.