patch 8.2.3520: cannot define a function for thesaurus completion

Problem:    Cannot define a function for thesaurus completion.
Solution:   Add 'thesaurusfunc'. (Yegappan Lakshmanan, closes #8987,
            closes 8950)
diff --git a/runtime/doc/quickref.txt b/runtime/doc/quickref.txt
index fcdd5a7..f7c5c17 100644
--- a/runtime/doc/quickref.txt
+++ b/runtime/doc/quickref.txt
@@ -948,6 +948,7 @@
 'textmode'	  'tx'	    obsolete, use 'fileformat'
 'textwidth'	  'tw'	    maximum width of text that is being inserted
 'thesaurus'	  'tsr'     list of thesaurus files for keyword completion
+'thesaurusfunc'	  'tsrfu'   function to be used for thesaurus completion
 'tildeop'	  'top'     tilde command "~" behaves like an operator
 'timeout'	  'to'	    time out on mappings and key codes
 'timeoutlen'	  'tm'	    time out time in milliseconds