NSDictionary dictionary vs init alloc vs new

Suppose I want to, for example, start creating key/value pairs using an NSMutableDictionary. I then seem to have at least three options for creating an empty, mutable dictionary with an unspecified capacity:

NSMutableDictionary *mutDict = [[NSMutableDictionary alloc] init]; // 1
NSMutableDictionary *mutDict = [NSMutableDictionary new];          // 2
NSMutableDictionary *mutDict = [NSMutableDictionary dictionary];   // 3

Now, as I understand it, [NSObject new] is practically (if not exactly?) the same as [[NSObject alloc] init]. So we can basically merge those two options as far as I'm concerned. Regarding the [NSDictionary dictionary] method though, the documentation says:

dictionary

Creates and returns an empty dictionary.

+ (id)dictionary

Return Value

A new empty dictionary.

Discussion

This method is declared primarily for use with mutable subclasses of NSDictionary.

If you don’t want a temporary object, you can also create an empty dictionary using alloc... and init.

I have a couple of questions regarding this documentation to begin with:

Firstly, why is it even declared in NSDictionary instead of NSMutableDictionary if that is where it is intended to be used?

Secondly, what do they mean by "temporary object" in this context?

In summary: Is there a difference between the third alternative above, compared to the first two? Could it have something to do wi