Document terminal buffer-local variables

Include an example of using them in 'statusline'.

runtime/doc/nvim_terminal_emulator.txt

@@ -10,6 +10,7 @@ Embedded terminal emulator				   *terminal-emulator*
 2. Spawning			|terminal-emulator-spawning|
 3. Input			|terminal-emulator-input|
 4. Configuration		|terminal-emulator-configuration|
+5. Status Variables		|terminal-emulator-status|
 
 ==============================================================================
 1. Introduction					     *terminal-emulator-intro*
@@ -112,5 +113,26 @@ There is also a corresponding |TermClose| event.
 The terminal cursor can be highlighted via |hl-TermCursor| and
 |hl-TermCursorNC|.
 
+==============================================================================
+5. Status Variables				    *terminal-emulator-status*
+
+Terminal buffers maintain some information about the terminal in buffer-local
+variables:
+
+- *b:term_title* The settable title of the terminal, typically displayed in
+  the window title or tab title of a graphical terminal emulator. Programs
+  running in the terminal can set this title via an escape sequence.
+- *b:terminal_job_id* The nvim job ID of the job running in the terminal. See
+  |job-control| for more information.
+- *b:terminal_job_pid* The PID of the top-level process running in the
+  terminal.
+
+These variables will have a value by the time the TermOpen autocmd runs, and
+will continue to have a value for the lifetime of the terminal buffer, making
+them suitable for use in 'statusline'. For example, to show the terminal title
+as the status line:
+>
+    :autocmd TermOpen * setlocal statusline=%{b:term_title}
+<
 ==============================================================================
  vim:tw=78:ts=8:noet:ft=help:norl: