ld: clarify comments on /DISCARD/ output section behavior

The previous comments made it difficult to understand how the /DISCARD/
output section interacts with non-contiguous regions.

In summary, the general rule is that the first (top-most) clause takes
precedence over subsequent ones:
- If /DISCARD/ appears first, the section is dropped. There is no need
  to warn about potential behavior changes with non-contiguous regions
  when the section is already discarded.
- If /DISCARD/ follows clauses that assign the input section to an output
  section, /DISCARD/ is ignored. If the input section can't be assigned
  to the output section for a later reason, an error will be raised.
  Otherwise the input section will be assigned as intended to an output
  section specified by one of the matching clauses previous to /DISCARD/.