patch 8.2.2605: Vim9: string index and slice does not include composing chars

Problem:    Vim9: string index and slice does not include composing chars.
Solution:   Include composing characters. (issue #6563)

runtime/doc/vim9.txt

@@ -96,8 +96,8 @@ script and `:def` functions; details are below:
 	def CallMe(count: number, message: string): bool
 - Call functions without `:call`: >
 	writefile(['done'], 'file.txt')
-- You cannot use `:xit`, `:t`, `:k`, `:append`, `:change`, `:insert` or
-  curly-braces names.
+- You cannot use `:xit`, `:t`, `:k`, `:append`, `:change`, `:insert`, `:open`
+  or curly-braces names.
 - A range before a command must be prefixed with a colon: >
 	:%s/this/that
 - Unless mentioned specifically, the highest |scriptversion| is used.
@@ -341,7 +341,8 @@ Functions can be called without `:call`: >
 Using `:call` is still possible, but this is discouraged.
 
 A method call without `eval` is possible, so long as the start is an
-identifier or can't be an Ex command.  Examples: >
+identifier or can't be an Ex command.  For a function either "(" or "->" must
+be following, without a line break.  Examples: >
 	myList->add(123)
 	g:myList->add(123)
 	[1, 2, 3]->Process()
@@ -696,8 +697,9 @@ for v:null.  When converting a boolean to a string "false" and "true" are
 used, not "v:false" and "v:true" like in legacy script.  "v:none" is not
 changed, it is only used in JSON and has no equivalent in other languages.
 
-Indexing a string with [idx] or [idx : idx] uses character indexes instead of
-byte indexes. Example: >
+Indexing a string with [idx] or taking a slice with [idx : idx] uses character
+indexes instead of byte indexes.  Composing characters are included.
+Example: >
 	echo 'bár'[1]
 In legacy script this results in the character 0xc3 (an illegal byte), in Vim9
 script this results in the string 'á'.
@@ -845,6 +847,8 @@ THIS IS STILL UNDER DEVELOPMENT - ANYTHING CAN BREAK - ANYTHING CAN CHANGE
 :enddef			End of a function defined with `:def`. It should be on
 			a line by its own.
 
+You may also find this wiki useful.  It was written by an early adoptor of
+Vim9 script: https://github.com/lacygoill/wiki/blob/master/vim/vim9.md
 
 If the script the function is defined in is Vim9 script, then script-local
 variables can be accessed without the "s:" prefix.  They must be defined