Clarify description of packw

[allenjbaum]

The description text says:
This instruction packs the low 16 bits of rs1 and rs2 into the 32 least-significant bits of rd,..
while the Sail snippet effective does the reverse, i.e.
This instruction packs the low 16 bits of rs2 and rs1 into the 32 least-significant bits of rd...

This is especially confusing because zext.h uses the packw encoding, except rs2 is x0 -
it's just pseudocode for packw rd, rs1, x0.
That doesn't make sense if rs1 and rs2 are reversed in the description
So, non-normative text that explains that it's pseudocode also wouldn't hurt.