patch 8.2.1588: cannot read back the prompt of a prompt buffer

Problem:    Cannot read back the prompt of a prompt buffer.
Solution:   Add prompt_getprompt(). (Ben Jackson, closes #6851)
diff --git a/runtime/doc/usr_41.txt b/runtime/doc/usr_41.txt
index ff223e6..b11f013 100644
--- a/runtime/doc/usr_41.txt
+++ b/runtime/doc/usr_41.txt
@@ -1118,6 +1118,7 @@
 	settagstack()		modify the tag stack of a window
 
 Prompt Buffer:					*promptbuffer-functions*
+	prompt_getprompt()	get the effective prompt text for a buffer
 	prompt_setcallback()	set prompt callback for a buffer
 	prompt_setinterrupt()	set interrupt callback for a buffer
 	prompt_setprompt()	set the prompt text for a buffer