[m-rev.] for post-commit review: explain grades more deeply

[Julien Fischer]

On Fri, 1 May 2020, Zoltan Somogyi wrote:

> Second, the user guide still mentions the --trail-segments option,
> and the compiler still has such an option. I propose that we delete
> all mention of --trail-segments option from both the user guide
> and the compiler, with one possible exception, that being that
> "trail-segments" would become an alternate name for the use_trail
> option. That would enable any obsolete reference in Mmakefiles
> to still work. Julien, is this accommodation of old Mmakefiles
> still needed? Has there been enough time since the old-style .tr grade
> has been retired to remove all references to trail segments?

No, there hasn't even been a stable release since the change that did
that.  For the next release (which is tenatively 20.06) we should keep
support for the --trail-segments option in place; after that release the
option can be dropped entirely.

Julien.