docs: clarify target.'cfg(...)' doesnt respect cfg from build script

[weihanglo]

What does this PR try to resolve?

This was a doc mistake, see #14306

How should we test and review this PR?

Read the doc.

Additional information

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[weihanglo]

This is not the first time we occurred to this error https://github.com/rust-lang/cargo/actions/runs/10116122648/job/27978291407?pr=14312:

---- doc::doc_cap_lints stdout ----
running `D:\a\cargo\cargo\target\debug\cargo.exe doc`
thread 'doc::doc_cap_lints' panicked at tests\testsuite\doc.rs:1862:10:

test failed running `D:\a\cargo\cargo\target\debug\cargo.exe doc`
error: process exited with code 101 (expected 0)
--- stdout

--- stderr
    Updating git repository `file:///D:/a/cargo/cargo/target/tmp/cit/t1269/a`
     Locking 2 packages to latest compatible versions
 Documenting a v0.5.0 (file:///D:/a/cargo/cargo/target/tmp/cit/t1269/a#c880d9f9)
    Checking a v0.5.0 (file:///D:/a/cargo/cargo/target/tmp/cit/t1269/a#c880d9f9)
 Documenting foo v0.0.1 (D:\a\cargo\cargo\target\tmp\cit\t1269\foo)

thread 'main' has overflowed its stack
error: could not document `foo`

Caused by:
  process didn't exit successfully: `rustdoc --edition=2015 --crate-type lib --crate-name foo src\lib.rs -o D:\a\cargo\cargo\target\tmp\cit\t1269\foo\target\doc --check-cfg cfg(docsrs) --check-cfg "cfg(feature, values())" --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat -C metadata=a2606d6191998d14 -L dependency=D:\a\cargo\cargo\target\tmp\cit\t1269\foo\target\debug\deps --extern a=D:\a\cargo\cargo\target\tmp\cit\t1269\foo\target\debug\deps\liba-361a7c653cc52c35.rmeta --crate-version 0.0.1` (exit code: 0xc00000fd, STATUS_STACK_OVERFLOW)

[weihanglo]

@bors try

[bors]

⌛ Trying commit 81147d1 with merge 4441ef6...

[bors]

☀️ Try build successful - checks-actions
Build commit: 4441ef6 (4441ef6b7cff27fe06c40d59c15168f555cd0311)

[epage]

@bors r+

[bors]

📌 Commit 828c7ef has been approved by epage

It is now in the queue for this repository.

[bors]

⌛ Testing commit 828c7ef with merge dc041a0...

[bors]

☀️ Test successful - checks-actions
Approved by: epage
Pushing dc041a0 to master...