Add internal documentation for all queries

[jyn514]

Currently, queries are inconsistently documented. Some have a doc-comment:

rust/compiler/rustc_middle/src/query/mod.rs

Lines 38 to 44 in b0f3940

	/// Return the span for a definition.
	/// Contrary to `def_span` below, this query returns the full absolute span of the definition.
	/// This span is meant for dep-tracking rather than diagnostics. It should not be used outside
	/// of rustc_middle::hir::source_map.
	query source_span(key: LocalDefId) -> Span {
	desc { "getting the source span" }
	}

while others only have the desc we mandate in the proc-macro:

rust/compiler/rustc_middle/src/query/mod.rs

Lines 26 to 30 in b0f3940

	query resolutions(_: ()) -> &'tcx ty::ResolverGlobalCtxt {
	eval_always
	no_hash
	desc { "getting the resolver outputs" }
	}

We should add documentation for all of these, then add #[warn(missing_docs)] so it doesn't regress in the future.

[Noratrieb]

#[warn(missing_docs)] isn't quite enough, we'd also need to remove this default here:

rust/compiler/rustc_macros/src/query.rs

Lines 235 to 236 in b0f3940

	let doc_string = format!("[query description - consider adding a doc-comment!] {}", doc_string);
	Ok(parse_quote! { #[doc = #doc_string] })