docs proj MG prefix mfix to projection

The following inputs can be set directly in the AMReX solver classes but we 

set them via the MFiX-Exa routines because we may want different inputs for the 

different solvers called by MFiX-Exa. 

NOTE: the nodal solver settings are read in directly by AMReX, 

the MAC and diffusion settings by MFiX. 

These control the nodal projection and must be preceded by "mfix":

These control the nodal projection and must be preceded by "projection": 

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|                         |  Description                                                          |   Type      | Default      |