Add some coments explaning the WhichTempDir type.

(That type maybe should just be an enum rather than a trait you implement... not
sure why I made it so general...)

Author: pnkfelix

tests/common/which_temp.rs

@@ -16,6 +16,12 @@ pub trait WhichTempDirectory {
 // Doing so has two drawbacks: 1. You need to clean-up the
 // generated directory yourself, and 2. You risk race conditions with
 // concurrent test runs.
+//
+// If you cange this to `GenerateIntoFreshTemp`, you get a fresh directory
+// (rooted at whatever is returned from `tempfile::tempdir()`). This reduces
+// race conditions (note that `cargo-bisect-rustc` still stores data in shared
+// locations like `~/.rustup`, so races can still arise) and allows the test
+// suite to clean up the directory itself.
 pub(crate) type WhichTempDir /*: WhichTempDirectory*/ = GenerateIntoFreshTemp;
 
 /// Using `GenerateIntoFreshTemp` yields a fresh directory in some