patch 9.0.1031: Vim9 class is not implemented yet

Problem:    Vim9 class is not implemented yet.
Solution:   Add very basic class support.
diff --git a/runtime/doc/vim9class.txt b/runtime/doc/vim9class.txt
index cb86b4d..b018a10 100644
--- a/runtime/doc/vim9class.txt
+++ b/runtime/doc/vim9class.txt
@@ -336,6 +336,9 @@
 A class is defined between `:class` and `:endclass`.  The whole class is
 defined in one script file.  It is not possible to add to a class later.
 
+A class can only be defined in a |Vim9| script file.  *E1315*
+A class cannot be defined inside a function.
+
 It is possible to define more than one class in a script file.  Although it
 usually is better to export only one main class.  It can be useful to define
 types, enums and helper classes though.
@@ -369,9 +372,9 @@
 							*implements*
 A class can implement one or more interfaces.
 							*specifies*
-A class can declare it's interface, the object members and methods, with a
+A class can declare its interface, the object members and methods, with a
 named interface.  This avoids the need for separately specifying the
-interface, which is often done an many languages, especially Java.
+interface, which is often done in many languages, especially Java.
 
 
 Defining an interface ~
@@ -634,7 +637,7 @@
 to allow that.  This helps writing code with fewer mistakes.
 
 
-Making object membes private with an underscore ~
+Making object members private with an underscore ~
 
 When an object member is private, it can only be read and changed inside the
 class (and in sub-classes), then it cannot be used outside of the class.