Fix common bug with if/then pattern

[jdesrosiers]

It's a very common bug to leave out the required keyword when using if/then. I recently realized that this site has the same error and is likely one of the reasons we see that problem so frequently. In this PR, I've fixed the bug and included some "note" blocks to explain why required is an important part of using this pattern.

I also noticed that the truth table has multiple problems

• Only half of the table renders. The first three rows get cut off.
• It's confusing. I found it difficult to understand whether "X" represented true or false.
• It's missing the T T T T case
• The "whole schema" value is incorrect in multiple places
• It doesn't take into account what happens if any of these keywords are not defined.

I we-wrote the truth table to take all of these things into account, but it ended up being rather large, so I condensed it into something that isn't quite a truth table, but is more clear. I did this by,

• Defining that a missing then or else has a default value of true.
• Using n/a to combine entries where a value had no effect on the result.

I used "T" and "F" to make the table a little more clear. Using an "X" and check mark (instead of empty space) would work as well, but I'm not familiar enough with the rst format to add special characters correctly.