Explain how '--' works in command-line invocation.

Excerpted from 'man bash': GNU Bash-2.05b, 2002 July 15.

For: RT #120732, as requested by bulk88.
op.c: Factor out two identical yyerror calls
Fix ‘Can't declare null operation in "my"’

This message is very unhelpful.  This was brought up in
ticket #115688.
Tests for rpeep optimisations in e45d8982ae