Auto merge of #9524 - ehuss:rustflags-compatibility, r=alexcrichton
Auto merge of #9524 - ehuss:rustflags-compatibility, r=alexcrichton

Add a note about rustflags compatibility.

Over time, Cargo occasionally starts issuing new flags that may conflict with flags the user is passing directly to the compiler. Some recent examples are `-C embed-bitcode` (which broke anyone passing `-Clto` manually), and `-C prefer-dynamic` (which is kind of a mess). Future conflicts might be things like `--remap-path-prefix` or `--extern-html-root-url` (for rustdoc). This adds a note to mention these potential conflicts.

Although we try to maintain backwards compatibility as much as possible throughout all of Cargo, this particular area I think is dangerous enough that it is prudent to have some kind of warning somewhere. It is very rare that conflicts arise in practice, but they can happen.

I also added a note about passing in flags that Cargo itself issues, which can cause problems. Closes #9358.

rustyhorde/cargo,eminence/cargo,rust-lang/cargo,rustyhorde/cargo,rust-lang/cargo,nipunn1313/cargo,nipunn1313/cargo,nipunn1313/cargo,eminence/cargo,rust-lang/cargo,rust-lang/cargo,rustyhorde/cargo,eminence/cargo,eminence/cargo,nipunn1313/cargo,rustyhorde/cargo