Clarify removal of AMREX_GIT_COMMIT_DEVELOP and AMREX_GIT_COMMIT_MASTER

the top-level MFix-Exa repo to see whether the AMReX submodule has new commits,

modified files, or untracked files.

To update the AMReX submodule reference by MFiX-Exa:

The CMake variables ``AMREX_GIT_COMMIT_DEVELOP`` and

``AMREX_GIT_COMMIT_DEVELOP`` have been removed. Instead, to update the AMReX

submodule referenced by MFiX-Exa:

.. code:: shell

    > git -C subprojects/amrex checkout UPDATED_AMREX_COMMIT_SHA1

    > git add subprojects/amrex

    > git commit -m 'Updating AMReX version'

This will only update the AMReX SHA-1 referenced by MFiX-Exa. Uncommitted AMReX

changes and untracked AMReX files under ``subprojects/amrex`` are not added by

``git add subprojects/amrex``. Change directories to ``subprojects/amrex`` and

commit any AMReX changes first before ``git add subprojects/amrex``.

``git add subprojects/amrex``. (To commit to the AMReX repo, change directories

to ``subprojects/amrex`` and run Git commands there, before ``git add

subprojects/amrex``.)

.. note::