Put limits on what makes synopsebot interested

Rationale for the numbers:

- All synopses are numbered with two digits, and there's no reason to suspect that'll change
- Biggest line number (5394) is from S03-operators.pod, and so 9999 is a comfortable maximum
- The first perl6 RT ticket had number #18400
- We're currently at RT ticket number #122716, and a quick estimate shows that the max is growing
  at around 5_000 a year; thus it'll take us ~15.5 years to reach 200_000