[master] update the doc for MasterToken::StringRegion to match the latest code.

the removed part was obsolete due to a later change (which is described
in the next paragraph of the doc).  keeping the sentence will just
confuse readers.

should be trivial enough, so I'm committing this at my discretion.

src/lib/dns/master_lexer.h

@@ -87,8 +87,7 @@ public:
     ///
     /// Any character can be stored in the valid range of the region.
     /// In particular, there can be a nul character (\0) in the middle of
-    /// the region.  On the other hand, it is not ensured that the string
-    /// is nul-terminated.  So the usual string manipulation API may not work
+    /// the region.  So the usual string manipulation API may not work
     /// as expected.
     ///
     /// The `MasterLexer` implementation ensures that there are at least