Remove comment about zero padding HMAC{256,384,512} keys. (dotnet#8761)

The zero padding is not observed in the `Key` property. It's an implementation detail of how HMAC works. To avoid any confusion, let's just remove this comment. We don't have it for HMACSHA1 or HMACMD5.

Author: vcsjones

xml/System.Security.Cryptography/HMACSHA256.xml

@@ -178,7 +178,7 @@
         <Parameter Name="key" Type="System.Byte[]" />
       </Parameters>
       <Docs>
-        <param name="key">The secret key for <see cref="T:System.Security.Cryptography.HMACSHA256" /> encryption. The key can be any length. However, the recommended size is 64 bytes. If the key is more than 64 bytes long, it is hashed (using SHA-256) to derive a 64-byte key. If it is less than 64 bytes long, it is padded to 64 bytes.</param>
+        <param name="key">The secret key for <see cref="T:System.Security.Cryptography.HMACSHA256" /> encryption. The key can be any length. However, the recommended size is 64 bytes. If the key is more than 64 bytes long, it is hashed (using SHA-256) to derive a 64-byte key.</param>
         <summary>Initializes a new instance of the <see cref="T:System.Security.Cryptography.HMACSHA256" /> class with the specified key data.</summary>
         <remarks>
           <format type="text/markdown"><![CDATA[  

xml/System.Security.Cryptography/HMACSHA384.xml

@@ -182,7 +182,7 @@
         <Parameter Name="key" Type="System.Byte[]" />
       </Parameters>
       <Docs>
-        <param name="key">The secret key for <see cref="T:System.Security.Cryptography.HMACSHA384" /> encryption. The key can be any length. However, the recommended size is 128 bytes. If the key is more than 128 bytes long, it is hashed (using SHA-384) to derive a 128-byte key. If it is less than 128 bytes long, it is padded to 128 bytes.</param>
+        <param name="key">The secret key for <see cref="T:System.Security.Cryptography.HMACSHA384" /> encryption. The key can be any length. However, the recommended size is 128 bytes. If the key is more than 128 bytes long, it is hashed (using SHA-384) to derive a 128-byte key.</param>
         <summary>Initializes a new instance of the <see cref="T:System.Security.Cryptography.HMACSHA384" /> class by using the specified key data.</summary>
         <remarks>
           <format type="text/markdown"><![CDATA[  

xml/System.Security.Cryptography/HMACSHA512.xml

@@ -182,7 +182,7 @@
         <Parameter Name="key" Type="System.Byte[]" />
       </Parameters>
       <Docs>
-        <param name="key">The secret key for <see cref="T:System.Security.Cryptography.HMACSHA512" /> encryption. The key can be any length. However, the recommended size is 128 bytes. If the key is more than 128 bytes long, it is hashed (using SHA-512) to derive a 128-byte key. If it is less than 128 bytes long, it is padded to 128 bytes.</param>
+        <param name="key">The secret key for <see cref="T:System.Security.Cryptography.HMACSHA512" /> encryption. The key can be any length. However, the recommended size is 128 bytes. If the key is more than 128 bytes long, it is hashed (using SHA-512) to derive a 128-byte key.</param>
         <summary>Initializes a new instance of the <see cref="T:System.Security.Cryptography.HMACSHA512" /> class with the specified key data.</summary>
         <remarks>
           <format type="text/markdown"><![CDATA[