Dev Docs Correction: CHECKMULTISIG Requires Sigs In Same Order As PubKeys

As reported by @gsalgado (thanks!), the docs incorrectly state that all
sigs are compared against all pubkeys.  This commit provides a corrected
description, additional details, and references in other parts of the
text where we mention multisig. (Fixes #622)
Merge branch 'checkmsig-order' (pull #623)