message: tweak MIRI_SYSROOT docs
diff: @@ -361,9 +361,10 @@ Moreover, Miri recognizes some environment variables:
  checkout. Note that changing files in that directory does not automatically
  trigger a re-build of the standard library; you have to clear the Miri build
  cache manually (on Linux, `rm -rf ~/.cache/miri`).
  -* `MIRI_SYSROOT` (recognized by `cargo miri` and the test suite)
  - indicates the sysroot to use. To do the same thing with `miri`
  - directly, use the `--sysroot` flag.
  +* `MIRI_SYSROOT` (recognized by `cargo miri` and the test suite) indicates the
  + sysroot to use. Only set this if you do not want to use the automatically
  + created sysroot. (The `miri` driver sysroot is controlled via the `--sysroot`
  + flag instead.)
  * `MIRI_TEST_TARGET` (recognized by the test suite) indicates which target
  architecture to test against. `miri` and `cargo miri` accept the `--target`
  flag for the same purpose.
  