ada: Implement representation aspect Max_Entry_Queue_Length

ada: Implement representation aspect Max_Entry_Queue_Length

Enforce Max_Entry_Queue_Length (and its
synonym Max_Entry_Queue_Depth) when applied to individual
protected entries.

gcc/ada/

	* exp_ch9.adb (Expand_N_Protected_Type_Declaration): Clarify
	comments.
	* sem_prag.adb (Analyze_Pragma): Check for duplicates
	Max_Entry_Queue_Length, Max_Entry_Queue_Depth and Max_Queue_Length
	for the same protected entry.
	* sem_util.adb (Get_Max_Queue_Length): Take into account all three
	representation aspects that can be used to set this restriction.
	(Has_Max_Queue_Length): Likewise.
	* doc/gnat_rm/implementation_defined_pragmas.rst:
	(pragma Max_Queue_Length): Fix pragma in example.
	* gnat_rm.texi: Regenerate.