doc: port prompt-buffer section (#14342)

[skip ci]

Changes from original include:
- "See |terminal-window|" -> "See |terminal|".
- Remove mention of using CTRL-W window commands in insert mode.
- Converted usage example to use the Nvim job and channel API.
- Removed logging from usage example, as ch_logfile() has no direct Nvim
  counterpart.
- Fixed some small grammar/spelling mistakes.

runtime/doc/channel.txt

@@ -174,4 +174,81 @@ Put this in `uppercase.vim` and run:  >
     nvim --headless --cmd "source uppercase.vim"
 
 ==============================================================================
+5. Using a prompt buffer				*prompt-buffer*
+
+If you want to type input for the job in a Vim window you have a few options:
+- Use a normal buffer and handle all possible commands yourself.
+  This will be complicated, since there are so many possible commands.
+- Use a terminal window.  This works well if what you type goes directly to
+  the job and the job output is directly displayed in the window.
+  See |terminal|.
+- Use a window with a prompt buffer. This works well when entering a line for
+  the job in Vim while displaying (possibly filtered) output from the job.
+
+A prompt buffer is created by setting 'buftype' to "prompt". You would
+normally only do that in a newly created buffer.
+
+The user can edit and enter one line of text at the very last line of the
+buffer.  When pressing Enter in the prompt line the callback set with
+|prompt_setcallback()| is invoked.  It would normally send the line to a job.
+Another callback would receive the output from the job and display it in the
+buffer, below the prompt (and above the next prompt).
+
+Only the text in the last line, after the prompt, is editable. The rest of the
+buffer is not modifiable with Normal mode commands.  It can be modified by
+calling functions, such as |append()|.  Using other commands may mess up the
+buffer.
+
+After setting 'buftype' to "prompt" Vim does not automatically start Insert
+mode, use `:startinsert` if you want to enter Insert mode, so that the user
+can start typing a line.
+
+The text of the prompt can be set with the |prompt_setprompt()| function. If
+no prompt is set with |prompt_setprompt()|, "% " is used. You can get the
+effective prompt text for a buffer, with |prompt_getprompt()|.
+
+The user can go to Normal mode and navigate through the buffer.  This can be
+useful to see older output or copy text.
+
+Any command that starts Insert mode, such as "a", "i", "A" and "I", will move
+the cursor to the last line.  "A" will move to the end of the line, "I" to the
+start of the line.
+
+Here is an example for Unix.  It starts a shell in the background and prompts
+for the next shell command.  Output from the shell is displayed above the
+prompt. >
+
+	" Function handling a line of text that has been typed.
+	func TextEntered(text)
+	  " Send the text to a shell with Enter appended.
+	  call chansend(g:shell_job, [a:text, ''])
+	endfunc
+
+	" Function handling output from the shell: Added above the prompt.
+	func GotOutput(channel, msg, name)
+	  call append(line("$") - 1, a:msg)
+	endfunc
+
+	" Function handling the shell exit: close the window.
+	func JobExit(job, status, event)
+	  quit!
+	endfunc
+
+	" Start a shell in the background.
+	let shell_job = jobstart(["/bin/sh"], #{
+		\ on_stdout: function('GotOutput'),
+		\ on_stderr: function('GotOutput'),
+		\ on_exit: function('JobExit'),
+		\ })
+
+	new
+	set buftype=prompt
+	let buf = bufnr('')
+	call prompt_setcallback(buf, function("TextEntered"))
+	call prompt_setprompt(buf, "shell command: ")
+
+	" start accepting shell commands
+	startinsert
+<
+
  vim:tw=78:ts=8:noet:ft=help:norl: