patch 8.1.0862: no verbose version of character classes

Problem:    No verbose version of character classes.
Solution:   Add [:ident:], [:keyword:] and [:fname:]. (Ozaki Kiichi,
            closes #1373)
diff --git a/runtime/doc/pattern.txt b/runtime/doc/pattern.txt
index b0c7f2e..f17db96 100644
--- a/runtime/doc/pattern.txt
+++ b/runtime/doc/pattern.txt
@@ -1118,6 +1118,9 @@
 *[:tab:]*	  [:tab:]		the <Tab> character
 *[:escape:]*	  [:escape:]		the <Esc> character
 *[:backspace:]*	  [:backspace:]		the <BS> character
+*[:ident:]*	  [:ident:]		identifier character (same as "\i")
+*[:keyword:]*	  [:keyword:]		keyword character (same as "\k")
+*[:fname:]*	  [:fname:]		file name character (same as "\f")
 	  The brackets in character class expressions are additional to the
 	  brackets delimiting a collection.  For example, the following is a
 	  plausible pattern for a UNIX filename: "[-./[:alnum:]_~]\+" That is,