[DOC] clearer description of -std= (try 2)

[Ralf Wildenhues]

* Joseph S. Myers wrote on Sun, Jan 20, 2008 at 01:04:55AM CET:
> 
> -ansi refers to fixed versions (c89 and c++98), not to a version that 
> should change in future, so it is not affected by changes in the compiler 
> default standard.

Ah ok, thank you for the clarification.

Cheers,
Ralf