Doc fix of 'gnus-fetch-old-headers'

* lisp/gnus/gnus-sum.el (gnus-fetch-old-headers): Avoid
treating 'some' and 'invisible' as symbols that need to be
hyperlinked.  Reported by Robert Pluim <rpluim@gmail.com>.
(Bug#33090)

lisp/gnus/gnus-sum.el

@@ -83,12 +83,12 @@ If an unread article in the group refers to an older, already
 read (or just marked as read) article, the old article will not
 normally be displayed in the Summary buffer.  If this variable is
 t, Gnus will attempt to grab the headers to the old articles, and
-thereby build complete threads.  If it has the value `some', all
-old headers will be fetched but only enough headers to connect
+thereby build complete threads.  If the value is the symbol `some',
+all old headers will be fetched but only enough headers to connect
 otherwise loose threads will be displayed.  This variable can
 also be a number.  In that case, no more than that number of old
-headers will be fetched.  If it has the value `invisible', all
-old headers will be fetched, but none will be displayed.
+headers will be fetched.  If the value is the symbol `invisible',
+all old headers will be fetched, but none will be displayed.
 
 The server has to support NOV for any of this to work.