bug#22554: 24.5; enriched-mode: C-S-a/S-Home doesn't mark to beginning of line

[Drew Adams <drew.adams@oracle.com>]

> After launching Emacs with ‘emacs -Q’, typing “C-x b newbuffer M-x
> enriched-mode”, then “||abcdefgfoo” followed by C-S-a or S-<Home>, point
> returns to the ‘a’ with nothing highlighted. In other modes, this key
> returns to the very beginning of the line starting with ‘|’ and with the
> line marked, as expected. C-S-e/S-<End> still works as expected in this
> and other modes. I could not find anything in the Enriched Text section
> of the Emacs manual that mentions this discrepancy.

Confirmed, in text mode.  Even just `C-a' returns to the `a' and not bol.
And this is the case at least as far back as Emacs 20.

`C-a' and `C-S-a' is bound in text mode to `beginning-of-line-text'
(not to `beginning-of-line').  The doc for that command does not
seem to cover the case of `|'.  It says only that it "moves past the
lef-margin indentation, the fill-prefix, and any indentation used
for centering or right-justifying the line".

The culprit seems to be `adaptive-fill-mode'.
(looking-at adaptive-fill-regexp) returns t.

The doc string should mention this part of the behavior, I think.

It seems that the point-moving behavior here is not a bug, but
there is a doc bug here.