Is there something like the official C documentation?
Strictly speaking, the C standard is not available for free. You need to purchase an electronic copy from http://www.ansi.org/ or http://www.iso.org.
However, you can download a free draft version from the Open Standards Organization's website - this is as good as or better than the official standard itself. It incorporates information from the ISO C99 standard plus three Technical Corrigenda, all of which are official ISO documents.
More info here: http://c-faq.com/ansi/avail.html
You can obtain a PDF copy of the C99 standard (ISO/IEC 9899:1999) from ANSI (and other fine standards organizations) for your private use for a modest fee - I believe it was 18 USD when I bought mine. Having that available is invaluable to me. But if you find a copy in public, then it is contraband.
You can find final committee drafts and current working documents of the C (C0x) Standard Committee at the JTC1, SC22, WG14 web site at the Open Standards Organization. (That's Joint Technical Committee 1, Sub-Committee 22, Working Group 14, I believe).
There is also a hardback book 'The C Standard: Incorporating Technical Corrigendum 1' (also including the Rationale) available for a slightly less modest 85 USD or thereabouts.
One of the best books about C is 'C: A Reference Manual' in its fifth edition. However, you asked for 'ultimate official source of standard C knowledge', and the only 'official' such source is the C standard, plus its corrigenda.
See also the material at http://www.coding-guidelines.com/ and http://c0x.coding-guidelines.com/.
There is this online website devdocs
They have multiple API documentations which are well organized. It is free and open-source.
For c doc click this link http://devdocs.io/c/
The official C99 standard is available for purchase from your national standards body; however, in practice everyone reads the draft with post-1999 corrections/amendments included, which you can download for free:
http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf