Docs for documentation tests should mention where doctests are picked up from (e.g. not from files in tests/*)

[robinst]

(@GuillaumeGomez asked me to open an issue here, see his comment: GuillaumeGomez/doc-comment#25 (comment))

The docs for documentation tests (https://doc.rust-lang.org/rustdoc/documentation-tests.html) don't currently mention in which files doctests can be put. E.g. they work if put into src/* files, but not in integration test files in tests/*. It would be good if that was mentioned there.

[GuillaumeGomez]

Actually, this should better be in the cargo book instead since it's a cargo-linked issue here.

[robinst]

Good point, maybe add a note here instead: https://doc.rust-lang.org/cargo/commands/cargo-test.html

Documentation tests are also run by default, which is handled by rustdoc. It extracts code samples from documentation comments and executes them. See the rustdoc book for more information on writing doc tests.

[jyn514]

@ehuss could you please move this to rust-lang/cargo?

[weihanglo]

Close as #10726 mentioning "doctests only applied to lib target" in more places (7860458, 2c9cfd6)