patch 8.0.1093: various small quickfix issues

Problem:    Various small quickfix issues.
Solution:   Remove ":" prefix from title set by a user. Add the qf_id2nr().
            function. Add a couple more tests.  Update documentation.
            (Yegappan Lakshmanan)
diff --git a/runtime/doc/quickfix.txt b/runtime/doc/quickfix.txt
index e555451..753f461 100644
--- a/runtime/doc/quickfix.txt
+++ b/runtime/doc/quickfix.txt
@@ -44,6 +44,18 @@
 The 'errorformat' option should be set to match the error messages from your
 compiler (see |errorformat| below).
 
+							*quickfix-ID*
+Each quickfix list has a unique identifier called the quickfix ID and this
+number will not change within a Vim session. The getqflist() function can be
+used to get the identifier assigned to a list.
+
+							*quickfix-ID*
+Each quickfix list has a unique identifier called the quickfix ID and this
+number will not change within a Vim session. The getqflist() function can be
+used to get the identifier assigned to a list. There is also a quickfix list
+number which may change whenever more than ten lists are added to a quickfix
+stack.
+
 						*location-list* *E776*
 A location list is a window-local quickfix list. You get one after commands
 like `:lvimgrep`, `:lgrep`, `:lhelpgrep`, `:lmake`, etc., which create a