get_mem_attrs documentation

[Richard Kenner]

    The documentation for get_mem_attrs is lacking explanations of the
    following arguments: alias, expr, offset, size, and align.

    Our coding standards require explicit documentation for each argument.
    Please add this information.  Please also indicate whether align is
    scaled in bits, bytes, and whether it is a power-of-two
    representation; that's often confusing.

The intent is that these are the same fields in the mem_attrs structure,
so they are documented there, but you are correct that duplication here
couldn't hurt and I will add them.