Fix documentation enabling flag on xsp-9999 ebuild

Currently, xsp-9999.ebuild's src_configure() passes '--doc' to the ./configure script, which assumes it's talking about --docdir (the default is fine).  I made the smallest possible change to the existing syntax (changed the && to an || and --doc to --disable-docs, which basically says "If the 'doc' USE flag is not present, add '--disable-docs' to the list of ./configure parameters").  Let me know if this is not ideal or needs to be rethought, but xsp-9999 merges now.