assign, documentation (PR#1434)

 In all the assignment operator expressions, `x' can be a name or
 an expression defining a part of an object to be replaced (e.g.,
 `z[[1]]').  The name does not need to be quoted, though it can be.

One improvement might be: "the name can be any string of text and may
be quoted, but this is only necessary if it is not a syntactically
valid name."

  I think the original point was that he expected that a[1] would be
  changed not that a new variable named "a[1]" would be created. I
  don't think your suggestion will help with this. I think that Brian
  is correct, unless of course you want to spend some amount of time
  figuring out the logic/parsing requirements.

  "Assign treats the quoted string as a symbol, it does not parse it in
  any way", might help.  On the otherhand, in some large number of
  years this is the first person who has had this problem, I'm not
  sure we should be documenting for less than 1% of the population 
  unless of course,
    "we are trying to be as accurate as possible without regard to
  readability"  :-), to quote the views of a certain member of R core
  on documentation.