Update the RMG following the release of 5.19.3

Reflow a paragraph with an odd newline, update a couple of previous commit
links (but keeping one as a major update (5.18->5.19) since it contains
more edits), update mention of the variables and files involved in the
Module::CoreList update, remove an out-dated reference to makerel adding
DOS line endings (it hasn't done that since 1f9a80b25f), and clarify that
CPAN's /src/README.html only lists stable releases.