runtime(doc): mention 'iskeyword' at :h charclass()

fixes: #15965

Signed-off-by: Christian Brabandt <cb@256bit.org>

Author: chrisbra

runtime/doc/builtin.txt

@@ -1,4 +1,4 @@
-*builtin.txt*	For Vim version 9.1.  Last change: 2024 Oct 31
+*builtin.txt*	For Vim version 9.1.  Last change: 2024 Nov 01
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -1625,7 +1625,7 @@ charclass({string})					*charclass()*
 		The character class is one of:
 			0	blank
 			1	punctuation
-			2	word character
+			2	word character (depends on 'iskeyword')
 			3	emoji
 			other	specific Unicode class
 		The class is used in patterns and word motions.