Add caveat about using empty extern blocks for linking

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Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
35/100
Issue type
Documentation
Clarity
Mostly clear
Activity status
Stale
Tech stack
rust
Domain
documentation

Research direction

Read the Link attribute section in src/items/external-blocks.md and review the linked discussion about Windows imported DLL function calls. Determine how empty extern blocks differ from putting the link attribute on each extern block, then add a concise caveat that accurately describes the distinction.

Written by the indexing model from the issue text.

Description

From The link attribute:

It is valid to add the link attribute on an empty extern block. You can use this to satisfy the linking requirements of extern blocks elsewhere in your code (including upstream crates) instead of adding the attribute to each extern block.

This is all true but gives the impression that using an empty extern block this way is equivalent to setting the link attribute on each extern block, which it isn't. E.g. see here. I think that it would be at least useful to note that functions in this scenario may be handled a bit differently.

But I'm struggling to reword it without sounding clumsy. Maybe it should have a short caveat tacked on the end?

Dominant language
Rust
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