update comment in tabulatedAccelerationSource

Information about the file structure were missing in the comments and the documentation seems outdated:

https://www.openfoam.com/documentation/cpp-guide/html/guide-fvoptions-sources-tabulated-acceleration.html

Usage
    Example usage:
    \verbatim
    SBM
    {
        type            tabulatedAccelerationSource;
        active          yes;

        timeDataFileName "constant/acceleration-terms.dat";
    }

    cat constant/acceleration-terms.dat:

    2
    (    t    acc      omega   dOmegadt
        (0.0 ((1 0 0) (0 0 0) (0 0 0)))
        (10  ((2 0 0) (0 0 0) (0 0 0)))
    )

    t = time [s] 
    acc = acceleration vector [m/s2]
    omega = rotation vector [rad/s]
    dOmegadt = angular acceleration [rad/s2]

    details see tabulatedAccelerationSourceTemplates.C

    \endverbatim

## Reattaching the author to the issue ticket: @Henning86 ##