Revert "spec:  change nesting order of strong/emph in ***a***."

This reverts commit 49a03b7666e2901d1ab2813fc0bdd23968d22979.
Special-case ***xx*** as strong/em.
Updated spec (but not yet examples) with new rules.

These reflect the current parsing algorithm.

We now get a symmetry that we lacked before:

    **a* b*
    *a *b**

are both emphasis within emphasis.

One asymmetry remains:

    **a*

has no emphasis, while

    *a**

has emphasis.  Further tweaking of the algorithm could regularize
this.