Changes are in the attached DIFF file (DISPLAY.DIFF).

As for the "overfull hbox", in the end I decided to simply add a
discretionary hyphen (@-) after 2nd hyphen (when I added it after
every hyphen, the line was broken after 2nd hyphen). Alternatively,
someone will have to rearrange the sentence.

As for Unicode code points it was decided to use "U+NNNN @sc{char
name}", so I adjusted all of them.

Additionally, I changed, in the last paragraph of "Visual Line Mode"
section, typesetting of the category char to use @samp{} instead of
``...''.

Finally, in a different file - FIXIT.TEXI - there is line 356, where
the word "dictionary" uses a discretionary hyphens - are they used
somewhere? Because, in the PDF there is no line breaking for this
sentence and if it's correct, perhaps adding this word to
DOCSTYLE.TEXI would be a better option.

--
S. U.