Document the strange behaviour of --all.

[doitian]

[RENDERED VIEW]

[doitian]

This is the verbose output from build and build --all

# /tmp/cargo-test ᚬdoc=
$ rm -rf target && cargo build --verbose && target/debug/foo
   Compiling lib1 v0.1.0 (/private/tmp/cargo-test/lib1)
     Running `rustc --edition=2018 --crate-name lib1 lib1/src/lib.rs --color always --crate-type lib --emit=dep-info,link -C debuginfo=2 -C metadata=f3df446d9af3fc59 -C extra-filename=-f3df446d9af3fc59 --out-dir /private/tmp/cargo-test/target/debug/deps -C incremental=/private/tmp/cargo-test/target/debug/incremental -L dependency=/private/tmp/cargo-test/target/debug/deps`
   Compiling lib2 v0.1.0 (/private/tmp/cargo-test/lib2)
     Running `rustc --edition=2018 --crate-name lib2 lib2/src/lib.rs --color always --crate-type lib --emit=dep-info,link -C debuginfo=2 -C metadata=3983a65eb823770d -C extra-filename=-3983a65eb823770d --out-dir /private/tmp/cargo-test/target/debug/deps -C incremental=/private/tmp/cargo-test/target/debug/incremental -L dependency=/private/tmp/cargo-test/target/debug/deps --extern lib1=/private/tmp/cargo-test/target/debug/deps/liblib1-f3df446d9af3fc59.rlib`
   Compiling foo v0.1.0 (/private/tmp/cargo-test)
     Running `rustc --edition=2018 --crate-name foo src/main.rs --color always --crate-type bin --emit=dep-info,link -C debuginfo=2 -C metadata=f6d4c13c0f1a2e63 -C extra-filename=-f6d4c13c0f1a2e63 --out-dir /private/tmp/cargo-test/target/debug/deps -C incremental=/private/tmp/cargo-test/target/debug/incremental -L dependency=/private/tmp/cargo-test/target/debug/deps --extern lib2=/private/tmp/cargo-test/target/debug/deps/liblib2-3983a65eb823770d.rlib`
    Finished dev [unoptimized + debuginfo] target(s) in 0.63s
foo 0

# /tmp/cargo-test ᚬdoc=
$ rm -rf target && cargo build --verbose --all && target/debug/foo
   Compiling lib1 v0.1.0 (/private/tmp/cargo-test/lib1)
     Running `rustc --edition=2018 --crate-name lib1 lib1/src/lib.rs --color always --crate-type lib --emit=dep-info,link -C debuginfo=2 --cfg 'feature="feature1"' -C metadata=2aa4e6ce16c4d8ec -C extra-filename=-2aa4e6ce16c4d8ec --out-dir /private/tmp/cargo-test/target/debug/deps -C incremental=/private/tmp/cargo-test/target/debug/incremental -L dependency=/private/tmp/cargo-test/target/debug/deps`
   Compiling lib2 v0.1.0 (/private/tmp/cargo-test/lib2)
     Running `rustc --edition=2018 --crate-name lib2 lib2/src/lib.rs --color always --crate-type lib --emit=dep-info,link -C debuginfo=2 -C metadata=7af8cdce6f9fbab9 -C extra-filename=-7af8cdce6f9fbab9 --out-dir /private/tmp/cargo-test/target/debug/deps -C incremental=/private/tmp/cargo-test/target/debug/incremental -L dependency=/private/tmp/cargo-test/target/debug/deps --extern lib1=/private/tmp/cargo-test/target/debug/deps/liblib1-2aa4e6ce16c4d8ec.rlib`
   Compiling foo v0.1.0 (/private/tmp/cargo-test)
     Running `rustc --edition=2018 --crate-name foo src/main.rs --color always --crate-type bin --emit=dep-info,link -C debuginfo=2 -C metadata=b39a504209d20d8f -C extra-filename=-b39a504209d20d8f --out-dir /private/tmp/cargo-test/target/debug/deps -C incremental=/private/tmp/cargo-test/target/debug/incremental -L dependency=/private/tmp/cargo-test/target/debug/deps --extern lib2=/private/tmp/cargo-test/target/debug/deps/liblib2-7af8cdce6f9fbab9.rlib`
    Finished dev [unoptimized + debuginfo] target(s) in 0.60s
foo 1