parse yada-yada only as a statement

parse yada-yada only as a statement

Commit f5727a1c71878a34f6255eb1a506c0b21af7d36f tried to make yada-yada
be parsed consistently as a term expression, but actually things are
more complicated than that.  The tokeniser didn't accept yada-yada in
the right contexts to make it usable as an expression, and changing
that would require decisions on resolving ambiguities between yada-yada
and flip-flop.  It's also documented as being a statement rather than
an expression, though with some incorrect information about ambiguities.

Overall it looks more like the intent was for yada-yada to be a statement.
This commit makes it grammatically treated as such, and also fixes up
the dubious parts of the documentation.  [perl #132150]