The doc/policy/ directory documents mempool policy but had no reference
for transaction standardness rules. This adds standardness.md covering
the checks in IsStandardTx(), AreInputsStandard(), IsWitnessStandard(),
and mempool acceptance, along with a table of configurable relay options.
Also improves the -datacarriersize help text in init.cpp to mention
the pre-v30.0 default of 83, making it easier for operators to restore
the previous OP_RETURN relay behavior.