Fill the region as one paragraph.
It removes any paragraph breaks in the region and extra newlines at the end,
indents and fills lines between the margins given by the
`current-left-margin' and `current-fill-column' functions.
(In most cases, the variable `fill-column' controls the width.)
It leaves point at the beginning of the line following the paragraph.
Normally performs justification according to the `current-justification'
function, but with a prefix arg, does full justification instead.
From a program, optional third arg JUSTIFY can specify any type of
justification. Fourth arg NOSQUEEZE non-nil means not to make spaces
between words canonical before filling. Fifth arg SQUEEZE-AFTER, if non-nil,
means don't canonicalize spaces before that position.
Return the `fill-prefix' used for filling.
If `sentence-end-double-space' is non-nil, then period followed by one space does not end a sentence, so don't break a line there.