Clarify that optional/required isn't relevant in ctor or indexer signatures

Fixes #782.

(I hadn't noticed that the same thing is relevant for indexers.)

Author: jskeet

standard/basic-concepts.md

@@ -525,8 +525,8 @@ The following accessibility constraints exist:
 Methods, instance constructors, indexers, and operators are characterized by their ***signatures***:
 
 - The signature of a method consists of the name of the method, the number of type parameters, and the type and parameter-passing mode of each of its formal parameters, considered in the order left to right. For these purposes, any type parameter of the method that occurs in the type of a formal parameter is identified not by its name, but by its ordinal position in the type parameter list of the method. The signature of a method specifically does not include the return type, parameter names, type parameter names, type parameter constraints, the `params` or `this` parameter modifiers, nor whether parameters are required or optional.
-- The signature of an instance constructor consists of the type and parameter-passing mode of each of its formal parameters, considered in the order left to right. The signature of an instance constructor specifically does not include the `params` modifier that may be specified for the right-most parameter.
-- The signature of an indexer consists of the type of each of its formal parameters, considered in the order left to right. The signature of an indexer specifically does not include the element type, nor does it include the `params` modifier that may be specified for the right-most parameter.
+- The signature of an instance constructor consists of the type and parameter-passing mode of each of its formal parameters, considered in the order left to right. The signature of an instance constructor specifically does not include the `params` modifier that may be specified for the right-most parameter, nor whether parameters are required or optional.
+- The signature of an indexer consists of the type of each of its formal parameters, considered in the order left to right. The signature of an indexer specifically does not include the element type, nor does it include the `params` modifier that may be specified for the right-most parameter, nor whether parameters are required or optional.
 - The signature of an operator consists of the name of the operator and the type of each of its formal parameters, considered in the order left to right. The signature of an operator specifically does not include the result type.
 - The signature of a conversion operator consists of the source type and the target type. The implicit or explicit classification of a conversion operator is not part of the signature.
 - Two signatures of the same member kind (method, instance constructor, indexer or operator) are considered to be the *same signatures* if they have the same name, number of type parameters, number of parameters, and parameter-passing modes, and an identity conversion exists between the types of their corresponding parameters ([§10.2.2](conversions.md#1022-identity-conversion)).