Update TSIG doc header to avoid godoc oddity

For some reason godoc treats it as plain text because it contains
"(TSIG)".
Bump up MaxCountScale in TestTXTRRQuick

Increases number of test iterations which should cover more corner-cases.
Test byte escaping in Domain Names and TXT strings

TestTXTRRQuick may not always cover these bytes so explicitly test that
they're covered.
Document domain name and TXT escaping behaviour