The files parsed by this function should contain only macro
  definitions; a warning will be issued if anything else other than
  comments or white space is found.
The macros argument may be a filename of a base set of macros,
  or the result of a previous call to loadRdMacros or
  loadPkgRdMacros in the same session.  These results should be
  assumed to be valid only within the current session.
The loadPkgRdMacros function first looks for an "RdMacros"
  entry in the package DESCRIPTION file.  If present, it should
  contain a comma-separated list of other package names; their macros
  will be loaded before those of the current package.  It will then look
  in the current package for .Rd files in the man/macros
  or help/macros subdirectories, and load those.