[Backport release-0.8] doc (#20616)

docs: various

- increase python line-length limit from 88 => 100.
- gen_help_html: fix bug in "tag" case (tbl_count => tbl_contains)

ref #15632
fix #18215
fix #18479
fix #20527
fix #20532

Co-authored-by: Ben Weedon <ben@weedon.email>
(cherry picked from commit f37cd2bb15)

Co-authored-by: Justin M. Keyes <justinkz@gmail.com>

src/nvim/terminal.c

@@ -111,9 +111,9 @@ struct terminal {
   //  - receive data from libvterm as a result of key presses.
   char textbuf[0x1fff];
 
-  ScrollbackLine **sb_buffer;       // Scrollback buffer storage for libvterm
-  size_t sb_current;                // number of rows pushed to sb_buffer
-  size_t sb_size;                   // sb_buffer size
+  ScrollbackLine **sb_buffer;       // Scrollback storage.
+  size_t sb_current;                // Lines stored in sb_buffer.
+  size_t sb_size;                   // Capacity of sb_buffer.
   // "virtual index" that points to the first sb_buffer row that we need to
   // push to the terminal buffer when refreshing the scrollback. When negative,
   // it actually points to entries that are no longer in sb_buffer (because the
@@ -154,7 +154,7 @@ static VTermScreenCallbacks vterm_screen_callbacks = {
   .movecursor  = term_movecursor,
   .settermprop = term_settermprop,
   .bell        = term_bell,
-  .sb_pushline = term_sb_push,
+  .sb_pushline = term_sb_push,  // Called before a line goes offscreen.
   .sb_popline  = term_sb_pop,
 };
 
@@ -952,7 +952,10 @@ static int term_bell(void *data)
   return 1;
 }
 
-// Scrollback push handler (from pangoterm).
+/// Scrollback push handler: called just before a line goes offscreen (and libvterm will forget it),
+/// giving us a chance to store it.
+///
+/// Code adapted from pangoterm.
 static int term_sb_push(int cols, const VTermScreenCell *cells, void *data)
 {
   Terminal *term = data;