notes on recent checkin

[Doug Evans]

fyi,

- I renamed the insn RELAX attribute to RELAXED (this was a FIXME)
- hacked on html.scm a bit
- added IDOC attribute to several ports so instructions can be sorted
  by kind (alu,branch,etc.) in the documentation
- split rtl traversal/evaluation out from rtl.scm into new file rtl-traverse.scm
- added new rtx-trim-for-doc function to turn fully specified rtl back into
  something more suitable to human reader
  (documentation should still be converted to pseudo-C, but this
  was simpler for now, and fun - boilerplate of how to parse rtl)
- added error checking to catch trying to redefine something already defined
  (would be useful to allow this, but it should require something like an
  "undef" or a "redefine")