policies/coding-style.md: move "defined" to other function-like things

[esyr]

While "defined" is indeed a unary preprocessor operator, it is treated as a function sintactically in the codebase, similar to sizeof and friends.

[levitte]

I think it should be mentioned that defined is a preprocessor operator and not a C keyword, for clarity

[esyr]

Not sure how to do it, the whole paragraph avoids mentioning the kind of the entities it discusses (and alignof and __attribute__ are not keywords anyway), what would you suggest?

[levitte]

Hmmm, I've understood alignof and __attribute__ as nonstandard C language keywords, just like sizeof. https://cppreference.com/w/c/keyword.html does at least refer to alignof as a keyword in C23.

defined is solely a preprocessor keyword, i.e. it's not at all defined in the C language proper.

I'll try to propose something.