patch 8.1.2080: the terminal API is limited and can't be disabled

Problem:    The terminal API is limited and can't be disabled.
Solution:   Add term_setapi() to set the function prefix. (Ozaki Kiichi,
            closes #2907)
diff --git a/runtime/doc/terminal.txt b/runtime/doc/terminal.txt
index c0caa67..ab26631 100644
--- a/runtime/doc/terminal.txt
+++ b/runtime/doc/terminal.txt
@@ -1,4 +1,4 @@
-*terminal.txt*	For Vim version 8.1.  Last change: 2019 Sep 20
+*terminal.txt*	For Vim version 8.1.  Last change: 2019 Sep 26
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -222,7 +222,7 @@
 					Vim width (no window left or right of
 					the terminal window) this value is
 					ignored.
-			++eof={text}	when using [range]: text to send after
+			++eof={text}	When using [range]: text to send after
 					the last line was written. Cannot
 					contain white space.  A CR is
 					appended.  For MS-Windows the default
@@ -234,6 +234,10 @@
 			++type={pty}	(MS-Windows only): Use {pty} as the
 					virtual console.  See 'termwintype'
 					for the values.
+			++api={expr}	Permit the function name starting with
+					{expr} to be called as |terminal-api|
+					function.  If {expr} is empty then no
+					function can be called.
 
 			If you want to use more options use the |term_start()|
 			function.
@@ -701,6 +705,15 @@
 			GetBufnr()->term_sendkeys(keys)
 
 
+term_setapi({buf}, {expr})				*term_setapi()*
+		Set the function name prefix to be used for the |terminal-api|
+		function in terminal {buf}.  For example: >
+		    :call term_setapi(buf, "Myapi_")
+		    :call term_setapi(buf, "")
+<
+		The default is "Tapi_".  When {expr} is an empty string then
+		no |terminal-api| function can be used for {buf}.
+
 term_setansicolors({buf}, {colors})			*term_setansicolors()*
 		Set the ANSI color palette used by terminal {buf}.
 		{colors} must be a List of 16 valid color names or hexadecimal
@@ -843,6 +856,9 @@
 				     color modes.  See |g:terminal_ansi_colors|.
 		   "tty_type"	     (MS-Windows only): Specify which pty to
 				     use.  See 'termwintype' for the values.
+		   "term_api"	     function name prefix for the
+				     |terminal-api| function.  See
+				     |term_setapi()|.
 
 		Can also be used as a |method|: >
 			GetCommand()->term_start()
@@ -902,9 +918,9 @@
 		Call a user defined function with {argument}.
 		The function is called with two arguments: the buffer number
 		of the terminal and {argument}, the decoded JSON argument. 
-		The function name must start with "Tapi_" to avoid
+		By default, the function name must start with "Tapi_" to avoid
 		accidentally calling a function not meant to be used for the
-		terminal API.
+		terminal API.  This can be changed with |term_setapi()|.
 		The user function should sanity check the argument.
 		The function can use |term_sendkeys()| to send back a reply.
 		Example in JSON: >