you are right, dangerous to write down specific values that might be wrongs hence leading to errors. 

But they just come from https://github.com/bcosca/fatfree#system-variables (who wrote this doc @github?)

+ extensive check in the Base::_construct() itself.

>  lead to the assumption that the code does nothing else than copying the value described here

it's what, in v.3.2, it exactly does, right ( https://github.com/bcosca/fatfree/blob/master/lib/base.php#L1625 )

**AND** it's a source of mistakes right

>  be changed before by the framework when running in CLI mode or while fixing any server configuration

so true

>  I vote for leaving this blank in the docs 

i understand all your valuables good reasons
the point is those are constants, and like all CONSTANTS they have predefined values, PHP.net doc them, aso 
even @bcosca writes 'Clearing global variable resets to default value', hence what are these 'default values' ?

But again, leaving this blank in the docs make sure there are no mistakes, better nothing than a mistake
Hence the dev can always check what the default value is on his setup. Yes, better that way.

and it's always possible to refer to the https://github.com/bcosca/fatfree#system-variables 
as I guess it's the file @bcosca updates when he makes [important] changes in the code