Toggle distinguishing between hard and soft newlines.
With a prefix argument ARG, enable the feature if ARG is
positive, and disable it otherwise. If called from Lisp, enable
it if ARG is omitted or nil.
When enabled, the functions `newline' and `open-line' add the
text-property `hard' to newlines that they insert, and a line is
only considered as a candidate to match `paragraph-start' or
`paragraph-separate' if it follows a hard newline.
When enabling, if there are newlines in the buffer but no hard
newlines, ask the user whether to mark as hard any newlines
preceding a `paragraph-start' line. From a program, second arg
INSERT specifies whether to do this; it can be `never' to change
nothing, t or `always' to force marking, `guess' to try to do the
right thing with no questions, nil or anything else to ask the
user.
Newlines not marked hard are called "soft", and are always internal
to paragraphs. The fill functions insert and delete only soft newlines.
Non-nil if Use-Hard-Newlines mode is enabled.
Use the command `use-hard-newlines' to change this variable.