Improve the EOS lesson

[vlisovsk]

The first section of the lesson says:

To retrieve a job outputfile, one can use three types of files:

But only two are listed right after.

The third should be a MassStorageFile which is considered to be almost deprecated, and some explanation should be added why should people avoid that.
Other options e.g. SharedFile exist and may be worth a brief mention here.

(Migrated from first-analysis-steps #221)