We often see in upgrades that new consensus flags' activations
have to be duplicated both in the mempool-flags and block-flags.
Though we sometimes do the opposite (adding flags that are only
active in blocks but NOT in mempool, such as was done in the
overhaul of D2468), that is rare in comparison.
Since STANDARD_SCRIPT_VERIFY_FLAGS contains all flags set by
GetNextBlockScriptFlags (besides SCRIPT_ENABLE_REPLAY_PROTECTION),
this produces no change in the current output of
GetStandardScriptFlags under any conditions.
At the same time, I've also updated the documentation of
STANDARD_SCRIPT_VERIFY_FLAGS to clarify why we need it.