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

Preview: http://dg2.dtrt.org/en/developer-reference#term-op-checkmultisig

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)

Note: the wiki has the same error, so I'll be updating that momentarily.