#52 warn when paragraph continuation might be a new bullet sublist

[Nathaniel Beaver]

Bullet lists require blank lines when starting a sublist, otherwise they will be interpreted as a paragraph continuation:

http://docutils.sourceforge.net/docs/ref/rst/restructuredtext.html#bullet-lists

The documentation notes that "Warnings may be issued by the implementation", but currently there is no error issued when inserting a blank line would result in a sublist (see attached example.rst).

[Günter Milde]

This is a generic problem for all lists:
"Blank lines separate paragraphs from each other and from other body
elements." (https://docutils.sourceforge.io/docs/ref/rst/restructuredtext.html#paragraphs)

"Blank lines may be omitted when the markup makes element
separation unambiguous, in conjunction with indentation."
(https://docutils.sourceforge.io/docs/ref/rst/restructuredtext.html#blank-lines)

The rationale is, that paragraphs might contain line wraps at
arbitrary places and valid output like

The module was last revised on May
3. 2021.

should pass without further intervention.

Which level should the message be:
Errors/Warnings will show up in the output (even if the output is as intended).
Info may be overlooked.

Do we want/tolerate the overhead (Docutils would need to check every paragraph line for valid
block-level markup).

See also the similar problem with field lists in https://sourceforge.net/p/docutils/feature-requests/80/.