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/ja_JP.UTF-8/PNG.txt

@@ -26,12 +26,12 @@ library.)
 Support for many colors allows photo-quality "rubber stamp" images to be used
 in Tux Paint, and alpha transparency allows for high-quality brushes.
 
-How To Make PNGs
+                           ### How To Make PNGs ###
 
 The following is a very brief list of ways to create PNGs or convert existing
 images into PNGs.
 
-GIMP & Krita
+→ GIMP & Krita ←
 
     Excellent tools with which to create PNG images for use in Tux Paint are
     GIMP and Krita, both high-quality Open Source interactive drawing and photo
@@ -43,10 +43,10 @@ GIMP & Krita
     /www.krita.org/, respectively.
 
 
+     
+→ Command-line Tools ←
 
-Command-line Tools
-
-    NetPBM
+    → NetPBM ←
 
         The Portable Bitmap tools (collectively known as "NetPBM") is a
         collection of Open Source command-line tools which convert to and from
@@ -58,8 +58,8 @@ Command-line Tools
         netpbm.sourceforge.net/.
 
 
-
-    cjpeg/djpeg
+         
+    → cjpeg/djpeg ←
 
         The "cjpeg" and "djpeg" command-line programs convert between the
         NetPBM Portable Any Map (PNM) format and JPEGs. It is possible that
@@ -68,11 +68,11 @@ Command-line Tools
         or to learn more, visit https://jpegclub.org/.
 
 
+         
 
 
-
-
-Proprietary Software for Windows
+     
+→ Proprietary Software for Windows ←
       + CorelDRAW (Corel) — http://www.corel.com/
       + Illustrator (Adobe) — http://www.adobe.com/products/illustrator.html
       + Paint Shop Pro (Corel) — https://www.paintshoppro.com/
@@ -80,13 +80,13 @@ Proprietary Software for Windows
       + PIXresizer (Bluefive software) — http://bluefive.pair.com/
         pixresizer.htm
 
-
-Macintosh Users
+     
+→ Macintosh Users ←
       + CorelDRAW (Corel) — http://www.corel.com/
       + GraphicConverter (Lemke Software) — https://www.lemkesoft.de/
         mac-fotobearbeitung-mac-diashow-mac-grafikprogramm-mac-bildbetrachter/
       + Illustrator (Adobe) — http://www.adobe.com/products/illustrator.html
       + Photoshop (Adobe) — http://www.adobe.com/products/photoshop.html
 
-
+