patch 8.0.1712: terminal scrollback is not limited

Problem:    Terminal scrollback is not limited.
Solution:   Add the 'terminalscroll' option.
diff --git a/runtime/doc/options.txt b/runtime/doc/options.txt
index cc19e25..0820699 100644
--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -1,4 +1,4 @@
-*options.txt*	For Vim version 8.0.  Last change: 2018 Mar 13
+*options.txt*	For Vim version 8.0.  Last change: 2018 Apr 14
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -7933,6 +7933,16 @@
 	Note that the "cterm" attributes are still used, not the "gui" ones.
 	NOTE: This option is reset when 'compatible' is set.
 
+						*'terminalscroll'* *'tlsl'*
+'terminalscroll' 'tlsl'	number	(default 10000)
+			global
+			{not in Vi}
+			{not available when compiled without the
+			|+terminal| feature}
+	Number of scrollback lines to keep.  When going over this limit the
+	first 10% of the scrollback lines are deleted.  This is just to reduce
+	the memory usage.  See |Terminal-Normal|.
+
 						*'termkey'* *'tk'*
 'termkey' 'tk'		string	(default "")
 			local to window
diff --git a/runtime/doc/terminal.txt b/runtime/doc/terminal.txt
index 63217c2..23e210d 100644
--- a/runtime/doc/terminal.txt
+++ b/runtime/doc/terminal.txt
@@ -288,7 +288,7 @@
 
 
 Terminal-Job and Terminal-Normal mode ~
-							*Terminal-mode*
+						*Terminal-mode* *Terminal-Job*
 When the job is running the contents of the terminal is under control of the
 job.  That includes the cursor position.  Typed keys are sent to the job.
 The terminal contents can change at any time.  This is called Terminal-Job
@@ -301,7 +301,9 @@
 Terminal-Job mode is where |:tmap| mappings are applied. Keys sent by
 |term_sendkeys()| are not subject to tmap, but keys from |feedkeys()| are.
 
-							*E946*
+It is not possible to enter Insert mode from Terminal-Job mode.
+
+						*Terminal-Normal* *E946*
 In Terminal-Normal mode you can move the cursor around with the usual Vim
 commands, Visually mark text, yank text, etc.  But you cannot change the
 contents of the buffer.  The commands that would start insert mode, such as
@@ -312,7 +314,10 @@
 the job ends while in Terminal-Normal mode this changes to
 "(Terminal-finished)".
 
-It is not possible to enter Insert mode from Terminal-Job mode.
+When the job outputs lines in the terminal, such that the contents scrolls off
+the top, those lines are remembered and can be seen in Terminal-Normal mode.
+The number of lines is limited by the 'terminalscroll' option. When going over
+this limit, the first 10% of the scrolled lins are deleted and are lost.
 
 
 Cursor style ~