docs: Fix up inline asm documentation

When writing the gcc-15/changes.html patch posted earlier, I've been
wondering where significant part of the Basic asm chapter went and the
problem was the insertion of a new @node in the middle of the Basic Asm
@node, plus not mentioning the new @node in the @menu.  So the asm constexpr
node was not normally visible and the Remarks for the section neither.

The following patch moves it before Asm Labels, removes the spots where it
described what hasn't been actually committed (constant expression can only
be a container with data/size member functions) and fixes up the toplevel
extended asm documentation (it was in the Basic Asm remarks and Extended Asm
section's remark still said it is not valid).

2025-01-16  Jakub Jelinek  <jakub@redhat.com>

	* doc/extend.texi (Using Assembly Language with C): Add Asm constexprs
	to @menu.
	(Basic Asm): Move @node asm constexprs before Asm Labels, rename to
	Asm constexprs, change wording so that it is clearer that the constant
	expression actually must not return a string literal, just some specific
	container and other wording tweaks.  Only talk about top-level for basic
	asms in this @node, move restrictions on top-level extended asms to ...
	(Extended Asm): ... here.