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].

[1]: https://gcc.gnu.org/pipermail/gcc-help/2023-November/143068.html
[2]: https://www.linuxfromscratch.org/lfs/view/12.0/chapter08/gcc.html
[3]: https://www.linuxfromscratch.org/blfs/view/12.0/general/gcc.html
[4]: https://gcc.gnu.org/pipermail/gcc-patches/2023-November/638760.html

gcc/ChangeLog:

	* doc/install.texi: Deem srcdir == objdir broken, but objdir
	as a subdirectory of srcdir fine.