Add a note about JS comparison operators

[jgonggrijp]

I would like to close all three of these tickets, because I believe they are based on a misunderstanding of the < operator, and I'd like to address this with a note in the documentation instead:

• Comparator edge-case handling #1768
• _.sortedIndex align with _.sortBy #1834
• Fixed bug in _.sortedIndex that put undefined at the start (FIX: #1834) #2042

The note would be somewhere at the bottom of the page, with a link to the MDN entry on the comparison operators. Reference entries on functions like _.sortBy and _.sortedIndex would reference to this note.

@jashkenas do you agree?

[jashkenas]

I agree! I don’t think that sorting NaN or undefined are an important problem to "fix" — you should never be sorting them.