enforce interface-functions to be external-declared

- libsolidity: Enforce interface-functions must be external-declared.
- Changelog adapted to reflect changes wrt. functions in interfaces.
- test: Adjustments according to prior interface-function changes.
- tests: Adapting SolidityEndToEndTest to interface-function change.
- docs: Adapted documentation to interface-function change.

test/libsolidity/syntaxTests/visibility/interface/function_default.sol

@@ -2,5 +2,4 @@ interface I {
 	function f();
 }
 // ----
-// Warning: (15-28): Functions in interfaces should be declared external.
-// Warning: (15-28): No visibility specified. Defaulting to "public". In interfaces it defaults to external.
+// TypeError: (15-28): Functions in interfaces must be declared external.

test/libsolidity/syntaxTests/visibility/interface/function_internal.sol

@@ -2,4 +2,4 @@ interface I {
 	function f() internal;
 }
 // ----
-// TypeError: (15-37): Functions in interfaces cannot be internal or private.
+// TypeError: (15-37): Functions in interfaces must be declared external.

test/libsolidity/syntaxTests/visibility/interface/function_private.sol

@@ -2,4 +2,4 @@ interface I {
 	function f() private;
 }
 // ----
-// TypeError: (15-36): Functions in interfaces cannot be internal or private.
+// TypeError: (15-36): Functions in interfaces must be declared external.

test/libsolidity/syntaxTests/visibility/interface/function_public.sol

@@ -2,4 +2,4 @@ interface I {
 	function f() public;
 }
 // ----
-// Warning: (15-35): Functions in interfaces should be declared external.
+// TypeError: (15-35): Functions in interfaces must be declared external.