Based on a discussion tonight on IRC with comments from @gmaxwell,
bitcoin428, and kadoban.
* Add a new introduction to the Standard Transaction section which
explains the purpose behind standard transactions so readers
understand that these aren't limitations for limitation's sake. More
specifically, note that bug-prevention isn't the only reason for
standard transactions---thanks to @gmaxwell for explaining the forward
compatablity reason to me.
* Make it clear that isStandard() only applies to loose transactions and
that it doesn't apply to txes in blocks. Thanks to bitcoin428 for
pointing out this source of confusion.
* Make it cleare that a non-standard redeemScript is a script that would
not pass isStandard(). Thanks to kadoban for pointing out the absense
of that information.
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