Where can I find a manual for my camera/flash/lens/etc. ? Christian Rollinger's "Canon FD documentation project" had most of them, but the site is no longer available. Some manuals and brochure downloaded from there are available in the Canon FD manuals page.
Can FD lenses be used on EOS cameras? Basically, they can't. There are several reasons. First, the EF mount has a very different coupling design, and its interface is totally electronic, while the FD one is mechanical. An EOS camera can't set the aperture on FD lenses because EF lenses have a motor inside the lens to move the aperture blades, thereby only stop-down mode can work.
Moreover, Canon increased the film-to-mount distance in the EF design. That implies that it's impossibile to achieve focus at infinity with a lens designed for a shorter distance - but adding some optics, in turn increasing the focal length, and decreasing image quality unless very good optics are used.
Canon itself designed only two adapters: one was a 1.2x extender to be used on high-end (and expensive) telephoto lenses, the other was a macro adapter. Others made and still make some adapter, but their quality does not look good, and, anyway, almost all features of an EOS camera are lost, stop-down is not the most versatile or faster mode. There are modification kits, but the lens will be no longer usable on FD cameras, and still they don't offer the features of a EF lens. Especially, recent EOS cameras don't have focusing screens designed for good manual focusing, but using Live View.
My advice: use FD lenses on FD cameras, and EF lenses on EOS ones.
Update: with the release of the EOS R mirrorless line of cameras, which use a shorter flange distance, it is possibile to use FD lenses with a simpler adapter. Because they also offer focus peaking within the EVF, they make simpler and faster using manual focusing lenses.
Can FL or R lenses be used on my FD camera? Yes, with a few exceptions. Some lenses have mechanical issues (i.e. a protruding rear element in some wide angle lenses like the 19mm) that do not allow them to be used on newer camera. Check your camera manual. Remember, anyway, that R and FL lenses do not allow for full aperture metering.
Update: RF lenses is a new mount (2018) derived from the EF one designed for the new EOS R cameras. They are totally different from the old R mount used by the very firsts Canon SLRs.