patch 8.1.1188: not all Vim variables require the v: prefix

Problem:    Not all Vim variables require the v: prefix.
Solution:   When scriptversion is 3 all Vim variables can only be used with
            the v: prefix.  (Ken Takata, closes #4274)
diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index c454f96..dba7486 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -1688,7 +1688,8 @@
 		When there are two counts, as in "3d2w", they are multiplied,
 		just like what happens in the command, "d6w" for the example.
 		Also used for evaluating the 'formatexpr' option.
-		"count" also works, for backwards compatibility.
+		"count" also works, for backwards compatibility, unless
+		|scriptversion| is 3 or higher.
 
 					*v:count1* *count1-variable*
 v:count1	Just like "v:count", but defaults to one when no count is
@@ -1720,7 +1721,8 @@
 	:silent! next
 	:if v:errmsg != ""
 	:  ... handle error
-<		"errmsg" also works, for backwards compatibility.
+<		"errmsg" also works, for backwards compatibility, unless
+		|scriptversion| is 3 or higher.
 
 				*v:errors* *errors-variable* *assert-return*
 v:errors	Errors found by assert functions, such as |assert_true()|.
@@ -2023,7 +2025,8 @@
 	:if v:shell_error
 	:  echo 'could not rename "foo" to "bar"!'
 	:endif
-<		"shell_error" also works, for backwards compatibility.
+<		"shell_error" also works, for backwards compatibility, unless
+		|scriptversion| is 3 or higher.
 
 					*v:statusmsg* *statusmsg-variable*
 v:statusmsg	Last given status message.  It's allowed to set this variable.
@@ -2123,7 +2126,8 @@
 v:this_session	Full filename of the last loaded or saved session file.  See
 		|:mksession|.  It is allowed to set this variable.  When no
 		session file has been saved, this variable is empty.
-		"this_session" also works, for backwards compatibility.
+		"this_session" also works, for backwards compatibility, unless
+		|scriptversion| is 3 or higher
 
 					*v:throwpoint* *throwpoint-variable*
 v:throwpoint	The point where the exception most recently caught and not
@@ -2154,7 +2158,7 @@
 v:version	Version number of Vim: Major version number times 100 plus
 		minor version number.  Version 5.0 is 500.  Version 5.1 (5.01)
 		is 501.  Read-only.  "version" also works, for backwards
-		compatibility.
+		compatibility, unless |scriptversion| is 3 or higher.
 		Use |has()| to check if a certain patch was included, e.g.: >
 			if has("patch-7.4.123")
 <		Note that patch numbers are specific to the version, thus both