bug#19490: Unclear info about libraries/packaging in manual

[Kelly Dean]

«(elisp) Library Headers» says:
⌜`Keywords'
     This line lists keywords for the `finder-by-keyword' help command.
     Please use that command to see a list of the meaningful keywords.
     ...
     The name of this field is unfortunate, since people often assume
     it is the place to write arbitrary keywords that describe their
     package, rather than just the relevant Finder keywords.⌝

«(elisp) Simple Packages» says:
⌜A simple package consists of a single Emacs Lisp source file.  The file
must conform to the Emacs Lisp library header conventions (*note
Library Headers)⌝

but it shows a contradicting example package with:
⌜;; Keywords: multimedia, frobnicate⌝

Also, «(elisp) Library Headers» says to use a ⌜Homepage⌝ header, but «(elisp) 
Simple Packages» says to use a ⌜URL⌝ header.