update and correct docs for OpenMP 3.x vs OpenMP 4.x and how to work around it.

**OpenMP Compiler compatibility info**\ :

Some compilers do not fully support the ``default(none)`` directive

and others (e.g. GCC version 9 and beyond) may implement OpenMP 4.0

semantics, which are incompatible with the OpenMP 3.1 semantics used

in LAMMPS (for maximal compatibility with compiler versions in use).

LAMMPS will try to detect compilers that use OpenMP 4.0 semantics and

change the directives accordingly, but if your compiler is not

detected, you may set the define ``-D LAMMPS_OMP_COMPAT=4`` in ``LMP_INC``

or the CMake build command.

and others (e.g. GCC version 9 and beyond, Clang version 10 and later)

may implement strict OpenMP 4.0 and later semantics, which are incompatible

with the OpenMP 3.1 semantics used in LAMMPS for maximal compatibility

with compiler versions in use.  If compilation with OpenMP enabled fails

because of your compiler requiring strict OpenMP 4.0 semantic, you can

change the behvior by adding ``-D LAMMPS_OMP_COMPAT=4`` to the ``LMP_INC``

variable in your makefile, or add it to the command line while configuring

with CMake. CMake will autodetect the suitable setting for the GNU, Clang,

and Intel compilers.

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