converted html tags to doxygen commands in documentation/src/fluid.dox

Note: could not convert <tt>.fl</tt> to \p .fl because it did not
      seem to give the desired format. similarly for .h .cxx and .fd



git-svn-id: file:///fltk/svn/fltk/branches/branch-1.3@6707 ea41ed52-d2ee-0310-a9c1-e6b18d33e121