(canonically-space-region): Doc fix.

(canonically-space-region): Doc fix.
(fill-context-prefix): If the second line has the first line prefix,
plus whitespace, use the part that the first line shares.
(fill-individual-paragraphs): When prefix changes,
usually get the new prefix from just one line,
with an exception for indented first lines of paragraphs.
Start a new paragraph when a line has extra indentation
after the fill prefix.