pronouns - Can 'all' be used as a predicative complement?

"But what are you going to do with it [= dragon’s egg] when it's hatched?" said Hermione.
"Well, I've bin doin' some readin', said Hagrid, pulling a large book from under his pillow. "Got this outta the library –– Dragon Breeding for Pleasure and Profit –– it's a bit outta date, o' course, but it's all in here. Keep the egg in the fire, 'cause their mothers breathe on ‘em, see, an' when it hatches, feed it on a bucket o' brandy mixed with chicken blood every half hour. An' see here –– how ter recognize diff'rent eggs –– what I got there's a Norwegian Ridgeback. They're rare, them."
(Harry Potter and the Sorcerer's Stone)

It looks like ‘all’ is a predicative complement over ‘it’. Yet I don’t find such usage in dictionaries. How do I see the ‘all’?

Answer

"It's all in here" means, approximately, "everything you need to know is contained in this work". The phrase is frequently used in response to a question or implicit question:

How do I install a grounded wall outlet? —(handing you a manual) It's all in here.
I can't find the 1927 commerce data. —(handing you a reference book) It's all in here.

As FumbleFingers very wittily and economically points out, whenever it heads a response to a question or implicit question it often refers to the answer:

Can 'all' be used as a predicative complement? —It's a mystery to me.

So the it Hagrid uses here is not co-referent with any previous it, but refers to the answer to the question which launched the discussion, "What are you going to do with it?"

As for all, McCawley 1998 has several pages (I'm sorry, I don't remember exactly where) on how all can be moved around. All may occur:

at the beginning, in the determiner position: All of it is here.
after the noun/pronoun it modifies, in an appositive-adjective position: It all is here.
after the verb, in an adverb position: It is all here.

But they all have the same meaning. Or all of them have same meaning. Or they have all the same meaning.