Syntax guide: Clarifying about 'other HTML elements'
Handle verbatim HTML in the procedure for identifying HTML tags

(rather than in the top-level procedure for identifying span tags)

This commit also makes the HTML tag that triggers the verbatim-html-mode
to be part of the verbatim html. Before this commit, the triggering tag was
identified as a HTML tag and the following text was identified as
verbatim-html.
Make HTML comment handling description sound similar to the other HTML tag handling stuff
Inline HTML: Consider the case when the HTML node is not the top node
Empty tags are better explained by linking to the HTML spec
Other HTML elements: Bring the spec up-to-date with the syntax guide

The syntax guide says "Other HTML elements can contain vfmd text, but
cannot be contained within span-level vfmd markup".

This commit enforces this restriction in the specification.
Syntax guide: Clarify about using verbatim HTML within blockquotes/lists
Group all HTML tag name lists under "Indentifying HTML tags"
When in verbatim-HTML-mode, block-level vfmd syntax shouldn't be recognized as well
When finding the end of paragraphs, account for backticks within HTML attributes

For example:
    <span attr="has ` in quotes"><pre><code attr="here ` too">

The previous algorithm would filter off the "<pre>" thinking that
it's within a code-span, but it actually isn't.

This commit handles this case correctly by switching priority between
the code-span detection algo and the HTML parsing algo depending on the
context.

Even with this commit, the algo does not handle backticks within
HTML attribute names, but it should be ok to not support that.

Fixes vfmd/vfmd-spec#1
Fix typo: Should use the plural 'characters'
Change the suggestion for implementers regarding HTML-aware end of paragraph

The suggestion that existed before this commit was unimplementable.