That is correct (and accurate), and that is how it is supposed to work:" 'DOCUMENT_ROOT’
The document root directory under which the current script is executing, as defined in the server’s configuration file."
IF you were to compile your own php, or otherwise manipulate you php.ini file (see the DH wiki), I suppose you could modify it, but there is certainly no need to do so in order to reference other paths in your script(s). It’s not apache that will be doing the “using” in this case anyway - it is PHP
You can always refer to your “outside of webspace” resources with absolute path references, or possibly by manipulating (truncating) other system “path” variables to “walk up” the directory tree (see the output of phpinfo() for likely candidates).
Sure they will (I do it all the time). If you set your permissions properly (DreamHost makes that easy, and secure, for you via the use of suexec) and refer to them properly, you can (and should) use them even if they are “outside” your “webroot”!