doc: Update the status of build directory not fully separated
Recently there are some people building GCC with srcdir == objdir and
the attempts just failed [1]. So stop to say "it should work". OTOH
objdir as a subdirectory of srcdir works: we've built GCC in LFS [2]
and BLFS [3] this way for decades and this is confirmed during the
review of a previous version of this patch [4].