(Help): Fix C-h a entry. Add C-h d entry.

(Help): Fix C-h a entry.  Add C-h d entry.
(Help Summary): Add C-h d and C-h e.
(Apropos): Clarify that all apropos commands may search for either
list of words or a regexp.  Add C-h d for apropos-documentation.
Describe apropos-documentation-sort-by-scores user option.