vim-patch:ffc89e4 runtime(doc): clarify complete_match() and 'isexpand' option

clarify complete_match() documentation to better explain its backward
search behavior, argument handling, and return value format and add an
example of isexpand

closes: https://github.com/vim/vim/pull/17212

ffc89e47d0

src/nvim/buffer_defs.h

@@ -558,7 +558,7 @@ struct file_buffer {
   char *b_p_fo;                 ///< 'formatoptions'
   char *b_p_flp;                ///< 'formatlistpat'
   int b_p_inf;                  ///< 'infercase'
-  char *b_p_ise;                ///< 'isexpand'
+  char *b_p_ise;                ///< 'isexpand' local value
   char *b_p_isk;                ///< 'iskeyword'
   char *b_p_def;                ///< 'define' local value
   char *b_p_inc;                ///< 'include'