Revamp documentation for _Complex types extension

Revamp documentation for _Complex types extension

While cleaning up the bug database, I noticed there was a request
to improve the documentation of the _Complex type extensions.
So I rewrote part of the documentation to make things clearer on
__real/__imag and even added documentation about casts between
the scalar and the complex type.
I moved the documentation of __builtin_complex under this section
too because it makes more sense than having it in the other
built-in section and reference it.

OK? Built make info and make html and checked out the results to
make sure the tables look decent.

gcc/ChangeLog:

	PR c/33193
	* doc/extend.texi: Extend the documentation about Complex
	types for casting and also rewrite the __real__/__imag__
	expression portion to use tables.
	Move __builtin_complex to the Complex type section.