Tweaks to w3m.sh to improve TXT version of docs

Make headers and description terms more apparent, by adding
some more regexps into the `sed` that the HTML is piped through
before being sent to `w3m`.

docs/w3m.sh

@@ -3,16 +3,49 @@
 infile=${@: -1}
 args=${@:1:${#}-1}
 
+# Add some visual indicators around some text
+# to make their context -- heading ("<h2>", "<h3", etc.),
+# description term ("<dt>"), etc -- more obvious in
+# the plain text version of the documentation:
+
+H2_OPEN_MUNGE="s/<h2\([^>]*\)>/<h2\1><center>### /"
+H2_CLOSE_MUNGE="s/<\/h2>/ ###<\/center><\/h2>/"
+H3_OPEN_MUNGE="s/<h3\([^>]*\)>/<h3\1>## /"
+H3_CLOSE_MUNGE="s/<\/h3>/ ##<\/h3>/"
+H4_OPEN_MUNGE="s/<h4\([^>]*\)>/<h4\1># /"
+H4_CLOSE_MUNGE="s/<\/h4>/ #<\/h4>/"
+DT_OPEN_MUNGE="s/<dt>/<dt>\&rarr; /"
+DT_CLOSE_MUNGE="s/<\/dt>/ \&larr;<\/dt>/"
+DD_CLOSE_MUNGE="s/<\/dd>/<br\/>\&nbsp;<br\/><\/dd>/"
+
 # FIXME: https://en.wikipedia.org/wiki/Line_breaking_rules_in_East_Asian_languages
 # | sed -e "s/\(.[、。）]\)/<nobr>\\1<\\/nobr>/g" \
 # This needs expanding & to not mess with <img> `alt` tag attributes or anything inside HTML tags (e.g., " quotes)!
 # -bjk 2023.07.17
 
 if [[ "$infile" =~ ja_JP ]]; then
-  sed -e "s/<\\/dd>/<br\\/><br\\/><\\/dd>/g" $infile \
+  sed $infile \
+    -e "$H2_OPEN_MUNGE" \
+    -e "$H2_CLOSE_MUNGE" \
+    -e "$H3_OPEN_MUNGE" \
+    -e "$H3_CLOSE_MUNGE" \
+    -e "$H4_OPEN_MUNGE" \
+    -e "$H4_CLOSE_MUNGE" \
+    -e "$DT_OPEN_MUNGE" \
+    -e "$DT_CLOSE_MUNGE" \
+    -e "$DD_CLOSE_MUNGE" \
     | php ./nobr_forbidden.php \
     | w3m $args
 else
-  sed -e "s/<\\/dd>/<br\\/><br\\/><\\/dd>/g" $infile \
+  sed $infile \
+    -e "$H2_OPEN_MUNGE" \
+    -e "$H2_CLOSE_MUNGE" \
+    -e "$H3_OPEN_MUNGE" \
+    -e "$H3_CLOSE_MUNGE" \
+    -e "$H4_OPEN_MUNGE" \
+    -e "$H4_CLOSE_MUNGE" \
+    -e "$DT_OPEN_MUNGE" \
+    -e "$DT_CLOSE_MUNGE" \
+    -e "$DD_CLOSE_MUNGE" \
     | w3m $args
 fi