rustdoc: Add reexport info

[GuillaumeGomez]

It looks like this (in the std page of Vec):

In the std::vec module:

So now come a few questions:

• Should we do it for local items too?
• What should we do for reexported doc(hidden) items?
• What should we go for reexported items within a doc(hidden) module as ancestor?
• Should we add an attribute to enable/disable this feature?

As this is likely going to need a lot of discussion, it's a draft for now. Nominating it for discussing it in next rustdoc meeting.

cc @rust-lang/rustdoc

[Kobzol]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: a22e66c (a22e66cf539b68a6349d5d13d40a9ccef7214160)
Base parent: 7022271 (70222712809cd5cc1718ed8995914a1cbacb6b92)

[rust-timer]

Finished benchmarking commit (a22e66c): comparison URL.

Overall result: ❌✅ regressions and improvements - no action needed

Benchmarking means the PR may be perf-sensitive. Consider adding rollup=never if this change is not fit for rolling up.

@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	0.2%	[0.2%, 0.2%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.4%	[-0.4%, -0.4%]	1
All ❌✅ (primary)	0.2%	[0.2%, 0.2%]	1

Max RSS (memory usage)

Results (secondary 5.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	5.0%	[5.0%, 5.0%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Cycles

Results (secondary 3.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	3.0%	[2.4%, 3.8%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

This perf run didn't have relevant results for this metric.

Bootstrap: 475.539s -> 473.775s (-0.37%)
Artifact size: 400.59 MiB -> 400.58 MiB (-0.00%)