This patch fixes the problem by joining the metavar terms with '|'.  So the help for the test case (adapted from an existing tuple test) looks like:

    usage: PROG [-h] W1 [W2 ...] [X1 [X2 ...]] Y1 Y2 Y3 [Z1]
    positional arguments:
      W1|W2       w
      X1|X2       x
      Y1|Y2|Y3    y
      Z1          z

Alternatives include:
- use ',',' ',or '/' instead of '|'
- use just the 1st item of the tuple
- use the default (non-tuple) metavar in the help line
These all pass existing tests.

The last alternative would use:

    #metavar = '|'.join(metavar)
    if len(metavar)>1:
        metavar = default
    else:
        metavar = metavar[0]