Clarify the call-site hierarchy and add a non-trivial example

[isHeSatoshi]

Fixes #1874.

The call-site hierarchy was described only with an example where every invocation is written in the source:

macro bar($i: ident) { $i }
macro foo($i: ident) { $i }

foo!(bar!(baz));

That gives ROOT -> baz, with no intermediate expansion, which makes the hierarchy hard to read: it is not obvious what is being tracked, and it is not obvious why a plain identifier like baz has a hierarchy at all when the section is about macro invocations.

This change:

• states what the hierarchy records: where code was written rather than how it was produced, with the invocation's SyntaxContext as the call_site link;
• notes that every token has one, because every token carries a syntax context, so naming baz in the notation just identifies whose hierarchy is being described;
• explains why the existing example is trivial: bar!(baz) is written as the argument to foo!, so it is not produced by foo!, and baz keeps the call site's context;
• adds the case that produces a non-trivial chain, an invocation written inside another macro's definition:

macro bar($i: ident) { $i }
macro foo() { bar!(baz) }

foo!();

I checked the second example against the compiler rather than reasoning about it. With -Zunpretty=expanded,hygiene on nightly, bar reports call_site_ctxt: #4 where #4 is the context created by foo's expansion (the first example reports call_site_ctxt: #0 for both invocations), and the baz in the output carries that same #4 context. Extending the chain to three macros gives call_site_ctxt #0, #4, #5 for a, b and c, with baz at #6, i.e. one fresh context per nesting level.

No new links are added; the section reuses the existing [callsite] and [sc] definitions.

[rustbot]

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