Fix missing PF_NO_MANA in get_pflag_desc()

While I was at it, I also moved the descriptions to list-player-flags.h
to keep things DRY and to hopefully prevent similar issues in the future.

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A little note: The second column could actually be removed and/or merged with the new third column. (In case of merge, I'm thinking some of the level information could be added, and perhaps some further descriptions could be added where we currently have NULL...?) Let me know if you'd prefer to remove the 2nd column or to have me merge the two columns (with adjustments for grammar, etc.)