patch 9.1.0199: Not enough tests for the slice() function

Problem:  Not enough tests for the slice() function.
Solution: Test with multibyte chars, and in both Legacy and Vim9 script.
          Update docs to be clearer about how it treats composing chars.
          (zeertzjq)

closes: #14275

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/builtin.txt b/runtime/doc/builtin.txt
index c857fbc..ec70220 100644
--- a/runtime/doc/builtin.txt
+++ b/runtime/doc/builtin.txt
@@ -1,4 +1,4 @@
-*builtin.txt*	For Vim version 9.1.  Last change: 2024 Mar 14
+*builtin.txt*	For Vim version 9.1.  Last change: 2024 Mar 23
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -9068,7 +9068,8 @@
 		Similar to using a |slice| "expr[start : end]", but "end" is
 		used exclusive.  And for a string the indexes are used as
 		character indexes instead of byte indexes, like in
-		|vim9script|.  Also, composing characters are not counted.
+		|vim9script|.  Also, composing characters are treated as a
+		part of the preceding base character.
 		When {end} is omitted the slice continues to the last item.
 		When {end} is -1 the last item is omitted.
 		Returns an empty value if {start} or {end} are invalid.
@@ -9465,8 +9466,8 @@
 		of byte index and length.
 		When {skipcc} is omitted or zero, composing characters are
 		counted separately.
-		When {skipcc} set to 1, Composing characters are ignored,
-		similar to  |slice()|.
+		When {skipcc} set to 1, composing characters are treated as a
+		part of the preceding base character, similar to |slice()|.
 		When a character index is used where a character does not
 		exist it is omitted and counted as one character.  For
 		example: >
@@ -9484,7 +9485,7 @@
 		in String {string}.
 		When {skipcc} is omitted or zero, composing characters are
 		counted separately.
-		When {skipcc} set to 1, Composing characters are ignored.
+		When {skipcc} set to 1, composing characters are ignored.
 		|strcharlen()| always does this.
 
 		Returns zero on error.