docs: fix: WARNING: Parsing of expression failed. Using fallback parser.

For the future, we can use https://sphinx-fortran.readthedocs.io/ for
Fortran functions which can have optional arguments. However, it's an
additional dependency.

Simplify the function declration.

gcc/fortran/ChangeLog:

	* doc/gfortran/intrinsic-procedures/aint.rst: Simplify function
	  declaration.
	* doc/gfortran/intrinsic-procedures/int.rst: Likewise.
	* doc/gfortran/intrinsic-procedures/anint.rst: Likewise.
	* doc/gfortran/intrinsic-procedures/char.rst: Likewise.
	* doc/gfortran/intrinsic-procedures/real.rst: Likewise.