pert:
You won't find any further documentation of the Arduino programming language anywhere else.
Actually, it's at the bottom of the first page.
Looking for something else?
See the libraries page for interfacing with particular types of hardware. Try the list of community-contributed code. The Arduino language is based on C/C++. It links against AVR Libc and allows the use of any of its functions; see its user manual for details.
It tells you specifically to look at C and C++. It would be easier to recommend rewording for this section instead of asking for a large amount of documentation that must be maintained.
pert:
Anything on the Forum is extremely difficult to find because the Forum is a huge disorganized mess.
I personally like stack overflow because users rate the value of answers, supports tags, allows user changes and comments. Maybe you can talking them into moving to a better forum.
Regards, Jon.