patch 8.1.2000: plugin cannot get the current IME status

Problem:    Plugin cannot get the current IME status.
Solution:   Add the getimstatus() function. (closes #4904)
diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index 56aa6a3..26a28dc 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -2468,6 +2468,7 @@
 getfsize({fname})		Number	size in bytes of file {fname}
 getftime({fname})		Number	last modification time of file
 getftype({fname})		String	description of type of file {fname}
+getimstatus()			Number	|TRUE| if the IME status is active
 getjumplist([{winnr} [, {tabnr}]])
 				List	list of jump list items
 getline({lnum})			String	line {lnum} of current buffer
@@ -5202,6 +5203,11 @@
 		Can also be used as a |method|: >
 			GetFilename()->getftype()
 
+getimstatus()						*getimstatus()*
+		The result is a Number, which is |TRUE| when the IME status is
+		active.
+		See 'imstatusfunc'.
+
 getjumplist([{winnr} [, {tabnr}]])			*getjumplist()*
 		Returns the |jumplist| for the specified window.