doc: Make tabline#show_buffers more prominent in documentation (#2663)

doc/airline.txt

@@ -1114,16 +1114,18 @@ are supported!
 
 * enable/disable enhanced tabline. (c) >
   let g:airline#extensions#tabline#enabled = 0
+<
+* enable/disable displaying buffers with a single tab. (c) >
+  let g:airline#extensions#tabline#show_buffers = 1
+
+  Note: With `show_buffers = 0`, tabs behave like the vim default tabline.
 
 * enable/disable displaying open splits per tab (only when tabs are opened) >
   let g:airline#extensions#tabline#show_splits = 1
 
 * switch position of buffers and tabs on splited tabline (c)
   (only supported for ctrlspace plugin). >
   let g:airline#extensions#tabline#switch_buffers_and_tabs = 0
-<
-* enable/disable displaying buffers with a single tab. (c) >
-  let g:airline#extensions#tabline#show_buffers = 1
 
 Note: If you are using neovim (has('tablineat') = 1), then you can click
 on the tabline with the left mouse button to switch to that buffer, and
@@ -1140,6 +1142,8 @@ with the middle mouse button to delete that buffer.
 * enable/disable displaying tabs, regardless of number. (c) >
   let g:airline#extensions#tabline#show_tabs = 1
 
+  Note: To display only tabs and no buffers, set `show_buffers=0` in addition.
+
 * enable/disable displaying number of tabs in the right side (c) >
   let g:airline#extensions#tabline#show_tab_count = 1