message: tweak alignment check docs
diff: @@ -165,9 +165,8 @@ up the sysroot. If you are using `miri` (the Miri driver) directly, see the
  Miri adds its own set of `-Z` flags:
  -* `-Zmiri-disable-alignment-check` disables checking pointer alignment. This is
  - useful to avoid [false positives][alignment-false-positives]. However, setting
  - this flag means Miri could miss bugs in your program.
  +* `-Zmiri-disable-alignment-check` disables checking pointer alignment, so you
  + can focus on other failures.
  * `-Zmiri-disable-stacked-borrows` disables checking the experimental
  [Stacked Borrows] aliasing rules. This can make Miri run faster, but it also
  means no aliasing violations will be detected.
  @@ -193,12 +192,13 @@ Miri adds its own set of `-Z` flags:
  default, alignment is checked by casting the pointer to an integer, and making
  sure that is a multiple of the alignment. This can lead to cases where a
  program passes the alignment check by pure chance, because things "happened to
  - be" sufficiently aligned. To avoid such cases, the symbolic alignment check
  - only takes into account the requested alignment of the relevant allocation,
  - and the offset into that allocation. This avoids such false negatives, but it
  - also incurs some false positives when the code does manual integer arithmetic
  - to ensure alignment. (The standard library `align_to` method works fine in
  - both modes; under symbolic alignment it only fills the middle slice when the
  + be" sufficiently aligned -- there is no UB in this execution but there would
  + be UB in others. To avoid such cases, the symbolic alignment check only takes
  + into account the requested alignment of the relevant allocation, and the
  + offset into that allocation. This avoids missing such bugs, but it also
  + incurs some false positives when the code does manual integer arithmetic to
  + ensure alignment. (The standard library `align_to` method works fine in both
  + modes; under symbolic alignment it only fills the middle slice when the
  allocation guarantees sufficient alignment.)
  * `-Zmiri-track-alloc-id=<id>` shows a backtrace when the given allocation is
  being allocated or freed. This helps in debugging memory leaks and
  @@ -211,8 +211,6 @@ Miri adds its own set of `-Z` flags:
  assigned to a stack frame. This helps in debugging UB related to Stacked
  Borrows "protectors".
  -[alignment-false-positives]: https://github.com/rust-lang/miri/issues/1074
  -
  Some native rustc `-Z` flags are also very relevant for Miri:
  * `-Zmir-opt-level` controls how many MIR optimizations are performed. Miri
  