Editorial: make enumerated attributes missing/invalid wording consistent

[keithamus]

Per #9944 (review) & #9944 (comment) there are several inconsistencies in the wording and ordering of enumerated attribute "missing value default" and "invalid value default" states.

• Some would say The <foo> attribute's while others The attribute's. Given the name of the attribute is redundant (as the prose always exists below the table and inside the heading for the attribute), it has been removed so they all read The attribute's.
• Some would say The attribute's missing value default and invalid value default are... while others would say The attribute's invalid value default and missing value default are.... These have been made consistent.
• Some would say The missing value default and *the* invalid value default, the redundant the has been removed.
• Some would separate missing/invalid into two sentences, others would separate by comma. They now all separate by comma.
• For enumerated attributes where both missing/invalid are the same, some read "are both the foo state." and others read "are the foo state.". This has been made consistent to "are both the".

Ordering "missing value default" before "invalid value default" makes them consistent based on the "Keywords an enumerated attributes" section which lists what to do about a missing attribute before it lists what to do with an invalid one.

Refs #9832.

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[annevk]

I'm not entirely sure about the insertion of the comma when "and" combines two clauses, but overall this is a huge improvement and I don't want to block on that. Thanks!