jQuery.merge: add note about it correcting the `length` property

[arthurvr]

Internally we use jQuery.merge to merge jQuery objects which are array-like so they need to have length corrected. I see it's not documented at http://api.jquery.com/jQuery.merge/ so perhaps an API ticket is needed (cc @arthurvr)

jquery/jquery#2155

[mgol]

This is documenting currently undocumented behavior so I'd like @jquery/core to approve it first.

[markelog]

We say that merge excepts arrays not array-likes, therefore we would need to get funky of explaining why jquery corrects the length.

Do we need to document that? Essentially, we would be documenting array-likes as acceptable arguments, if user uses merge correctly, their wouldn't even know about that side-effect.

[mgol]

Do we need to document that? Essentially, we would be documenting array-likes as acceptable arguments, if user uses merge correctly, their wouldn't even know about that side-effect.

The problem is jQuery.merge is public and de facto accepts array-likes; people use it to e.g. merge with the arguments object. So we might as well document this behavior, I doubt it'll ever change.

[timmywil]

we might as well document this behavior, I doubt it'll ever change.

Agreed. However, although we use this term in several places already, I think we should document what exactly "Array-like" means, perhaps on the types page.

[mgol]

@timmywil Good idea.

[arthurvr]

Once the Array-like type lands to the types page I can update this PR. Thanks for the suggestions you all!