Add a note about intelligent HTML parsing

A `<` at some point in the paragraph should not make
us think that the rest of the document is a HTML tag.
If a paragraph contains a HTML comment, don't wrap it <p> tags
Handle badly formed links correctly

For example:

 1. [link text](just text)
 2. [link text](just "some" "text")
 3. [link text][unclosed
 4. [link text](unclosed

For the above inputs, this commit makes them not get recognized as
links, but instead looks for the definition of [link text] in a
reference-resolution block.
Need to remove escaping backslashes from link titles too
reference-resolution blocks can appear in nested constructs too

Just making that explicit.
Make it explicit that matching a regexp implies partial matching
While de-escaping, only escaping backslashes should be removed

For example, `a\\()b` should be de-escaped to `a\()b`, and not to `a()b`
userguide: Lists
userguide: Cleanup end-of-line whitespace
userguide: Paragraphs
userguide: Suppress types of headers from the ToC
userguide: Code-blocks can be formed by indenting with a tab too
Change attribute quoting character to double-quotes.

To be consistent with the other Markdown variants, including Markdown.pl
userguide: Links
userguide: Emphasis
userguide: Code spans
userguide: Images
userguide: Automatic links
userguide: Automatic escaping for HTML output
userguide: Backslash escapes