patch 8.2.3906: Vim9 help still contains "under development" warnings

Problem:    Vim9 help still contains "under development" warnings.
Solution:   Remove the explicit warning.

runtime/doc/vim9.txt

@@ -1,17 +1,14 @@
-*vim9.txt*	For Vim version 8.2.  Last change: 2021 Dec 22
+*vim9.txt*	For Vim version 8.2.  Last change: 2021 Dec 26
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
 
 
-THIS IS STILL UNDER DEVELOPMENT - ANYTHING CAN BREAK - ANYTHING CAN CHANGE
-
 Vim9 script commands and expressions.			*Vim9* *vim9*
 
 Most expression help is in |eval.txt|.  This file is about the new syntax and
 features in Vim9 script.
 
-THIS IS STILL UNDER DEVELOPMENT - ANYTHING CAN BREAK - ANYTHING CAN CHANGE
 
 
 1.  What is Vim9 script?		|Vim9-script|
@@ -27,8 +24,6 @@ THIS IS STILL UNDER DEVELOPMENT - ANYTHING CAN BREAK - ANYTHING CAN CHANGE
 
 1. What is Vim9 script?					*Vim9-script*
 
-THIS IS STILL UNDER DEVELOPMENT - ANYTHING CAN BREAK - ANYTHING CAN CHANGE
-
 Vim script has been growing over time, while preserving backwards
 compatibility.  That means bad choices from the past often can't be changed
 and compatibility with Vi restricts possible solutions.  Execution is quite
@@ -76,8 +71,6 @@ rewrite old scripts, they keep working as before.  You may want to use a few
 
 2. Differences from legacy Vim script			*vim9-differences*
 
-THIS IS STILL UNDER DEVELOPMENT - ANYTHING CAN BREAK - ANYTHING CAN CHANGE
-
 Overview ~
 
 Brief summary of the differences you will most often encounter when using Vim9
@@ -354,7 +347,9 @@ And with autocommands: >
 Although using a :def function probably works better.
 
 Declaring a variable with a type but without an initializer will initialize to
-zero, false or empty.
+false (for bool), empty (for string, list, dict, etc.) or zero (for number,
+any, etc.).  This matters especially when using the "any" type, the value will
+default to the number zero.
 
 In Vim9 script `:let` cannot be used.  An existing variable is assigned to
 without any command.  The same for global, window, tab, buffer and Vim
@@ -1067,8 +1062,6 @@ Using ++var or --var in an expression is not supported yet.
 
 3. New style functions					*fast-functions*
 
-THIS IS STILL UNDER DEVELOPMENT - ANYTHING CAN BREAK - ANYTHING CAN CHANGE
-
 							*:def*
 :def[!] {name}([arguments])[: {return-type}]
 			Define a new function by the name {name}.  The body of
@@ -1184,8 +1177,6 @@ for each closure call a function to define it: >
 
 4. Types					*vim9-types*
 
-THIS IS STILL UNDER DEVELOPMENT - ANYTHING CAN BREAK - ANYTHING CAN CHANGE
-
 The following builtin types are supported:
 	bool
 	number
@@ -1366,8 +1357,6 @@ Same for |extend()|, use |extendnew()| instead, and for |flatten()|, use
 5. Namespace, Import and Export
 					*vim9script* *vim9-export* *vim9-import*
 
-THIS IS STILL UNDER DEVELOPMENT - ANYTHING CAN BREAK - ANYTHING CAN CHANGE
-
 A Vim9 script can be written to be imported.  This means that everything in
 the script is local, unless exported.  Those exported items, and only those
 items, can then be imported in another script.
@@ -1682,7 +1671,8 @@ Specific items from TypeScript we avoid:
 - TypeScript can use an expression like "99 || 'yes'" in a condition, but
   cannot assign the value to a boolean.  That is inconsistent and can be
   annoying.  Vim recognizes an expression with && or || and allows using the
-  result as a bool.  TODO: to be reconsidered
+  result as a bool.  The |falsy-operator| was added for the mechanism to use a
+  default value.
 - TypeScript considers an empty string as Falsy, but an empty list or dict as
   Truthy.  That is inconsistent.  In Vim an empty list and dict are also
   Falsy.

src/version.c

@@ -749,6 +749,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3906,
 /**/
     3905,
 /**/