Document fast fieldsfile loading.

> if you feel strongly state that that assumptions are made

I'm not too keen on an exact description, as full details would just be complex and tedious.
For example, we are obviously ignoring LBEGIN / LBNREC  etc., but also more questionable cases like LBSRCE, LBEXP, LBFCFC, LBPROJ, LBTYP and "unused" parts of LBUSER / LBRSVD. Quite apart from worrying about what it should "ideally" do, describing all this precisely would be excessive -- even just to list what we do use + how.

For once, I prefer to be vague!!