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

test/old/testdir/gen_opt_test.vim

@@ -261,6 +261,7 @@ let test_values = {
       "\ 'imactivatekey': [['', 'S-space'], ['xxx']],
       \ 'isfname': [['', '@', '@,48-52'], ['xxx', '@48']],
       \ 'isident': [['', '@', '@,48-52'], ['xxx', '@48']],
+      \ 'isexpand': [['', '.,->', '/,/*,\\,'], [',,', '\\,,']],
       \ 'iskeyword': [['', '@', '@,48-52'], ['xxx', '@48']],
       \ 'isprint': [['', '@', '@,48-52'], ['xxx', '@48']],
       \ 'jumpoptions': [['', 'stack'], ['xxx']],