git bisect docs: formatting tweaks

``A`` is the first commit in LLVM ever, ``97724f18c79c``.

``B`` is the first commit in MLIR, ``aed0d21a62db``.  ``D`` is the merge commit

that merged MLIR into the main LLVM repository, ``0f0d0ed1c78f``. ``C`` is the

last commit in MLIR before it got merged, ``0f0d0ed1c78f^2``. (The ``^n``

modifier selects the n'th parent of a merge commit.)

``B`` is the first commit in MLIR, ``aed0d21a62db``.

``D`` is the merge commit that merged MLIR into the main LLVM repository,

``0f0d0ed1c78f``.

``C`` is the last commit in MLIR before it got merged, ``0f0d0ed1c78f^2``. (The

``^n`` modifier selects the n'th parent of a merge commit.)

``git bisect`` goes through all parent revisions. Due to the way MLIR was

merged, at every revision at ``C`` or earlier, *only* the ``mlir/`` directory

The best workaround is to pass a list of directories to ``git bisect``:

If you know the bug is due to a change in llvm, clang, or compiler-rt, use

``git bisect start -- clang llvm compiler-rt``. That way, the commits in

``mlir`` are never evaluated.

  .. code-block:: bash

     git bisect start -- clang llvm compiler-rt

That way, the commits in ``mlir`` are never evaluated.

Alternatively, ``git bisect skip aed0d21a6 aed0d21a6..0f0d0ed1c78f`` explicitly

skips all commits on that branch. It takes 1.5 minutes to run on a fast