patch 8.2.0590: no 'backspace' value allows ignoring the insertion point

Problem:    No 'backspace' value allows ignoring the insertion point.
Solution:   Add the "nostop" and 3 values. (Christian Brabandt, closes #5940)
diff --git a/runtime/doc/options.txt b/runtime/doc/options.txt
index 9410726..baccd25 100644
--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -915,6 +915,8 @@
 	eol	allow backspacing over line breaks (join lines)
 	start	allow backspacing over the start of insert; CTRL-W and CTRL-U
 		stop once at the start of insert.
+	nostop	like start, except CTRL-W and CTRL-U do not stop at the start of
+		insert.
 
 	When the value is empty, Vi compatible backspacing is used.
 
@@ -923,6 +925,7 @@
 	  0	same as ":set backspace=" (Vi compatible)
 	  1	same as ":set backspace=indent,eol"
 	  2	same as ":set backspace=indent,eol,start"
+	  3	same as ":set backspace=indent,eol,nostop"
 
 	See |:fixdel| if your <BS> or <Del> key does not do what you want.
 	NOTE: This option is set to "" when 'compatible' is set.