GC options

[Alex]

I'm experimenting with GC, and reading
https://dlang.org/spec/garbage.html#gc_config

There is an option setting possible via
´´´
extern(C) __gshared string[] rt_options = [ "gcopt=gc:precise" ];
//gc:conservative|precise|manual
´´´
conservative and manual seem to work, while setting gc to 
precise, yields the error:

"No GC was initialized, please recheck the name of the selected 
GC ('precise')."

Is the docu out of date, or do I something wrong?
And another question: is there some extended docu, what do the 
options lead to?