more documentation on the process of loading source and compiled files

more documentation on the process of loading source and compiled files

* doc/ref/api-evaluation.texi (Load Paths): Move documentation of
  %load-path and related procedures here, from Build Config.  Add docs
  for %load-compiled-path.

* doc/ref/api-foreign.texi:
* doc/ref/api-modules.texi:
* doc/ref/api-options.texi:
* doc/ref/scheme-using.texi: Update xrefs.