patch 8.2.2854: custom statusline cannot contain % items

Problem:    Custom statusline cannot contain % items.
Solution:   Add "%{% expr %}". (closes #8190)

runtime/doc/options.txt

@@ -7339,6 +7339,18 @@ A jump table for the options with a short description can be found at |Q_op|.
 	      Note that there is no '%' before the closing '}'.  The
 	      expression cannot contain a '}' character, call a function to
 	      work around that.  See |stl-%{| below.
+	{% -  This is almost same as { except the result of the expression is
+	      re-evaluated as a statusline format string.  Thus if the
+	      return value of expr contains % items they will get expanded.
+	      The expression can contain the } character, the end of
+	      expression is denoted by %}.
+	      The For example: >
+		func! Stl_filename() abort
+		    return "%t"
+		endfunc
+<	        `stl=%{Stl_filename()}`   results in `"%t"`
+	        `stl=%{%Stl_filename()%}` results in `"Name of current file"`
+	} -   End of `{%` expression
 	( -   Start of item group.  Can be used for setting the width and
 	      alignment of a section.  Must be followed by %) somewhere.
 	) -   End of item group.  No width fields allowed.