Refine nonblock receive/write return types per review

Per review feedback:

- `SSLSocket#syswrite_nonblock`:
  - Use `string` (= `String | _ToStr`) instead of `_ToS` for the
    argument: the C implementation uses `StringValue`, which converts
    via `to_str`, so `nil`, `Hash`, etc. are not accepted.
  - Remove `nil` from the `exception: false` return: unlike
    `sysread_nonblock`, write has no EOF-equivalent path that returns
    `nil`; it raises on real errors and only converts wait-state into
    `:wait_readable` / `:wait_writable`.
- `BasicSocket#recv_nonblock`, `BasicSocket#recvmsg_nonblock`,
  `Socket#recvfrom_nonblock`, `UDPSocket#recvfrom_nonblock`: include
  `nil` in both `exception: true` and `exception: false` overloads.
  The rdoc states that these methods return `nil` when the underlying
  `recvfrom(2)` returns 0 (closed connection or empty datagram),
  regardless of `exception:`.

Author: zonuexe

stdlib/openssl/0/openssl.rbs

@@ -9342,8 +9342,8 @@ module OpenSSL
       # Writes *string* to the SSL connection in a non-blocking manner.  Raises an
       # SSLError if writing would block.
       #
-      def syswrite_nonblock: (_ToS string, ?exception: true) -> Integer
-                           | (_ToS string, exception: false) -> (Integer | :wait_readable | :wait_writable | nil)
+      def syswrite_nonblock: (string, ?exception: true) -> Integer
+                           | (string, exception: false) -> (Integer | :wait_readable | :wait_writable)
 
       # <!--
       #   rdoc-file=ext/openssl/lib/openssl/ssl.rb

stdlib/socket/0/basic_socket.rbs

@@ -330,8 +330,8 @@ class BasicSocket < IO
   # ### See
   # *   Socket#recvfrom
   #
-  def recv_nonblock: (Integer maxlen, ?Integer flags, ?String buf, ?exception: true) -> String
-                   | (Integer maxlen, ?Integer flags, ?String buf, exception: false) -> (String | :wait_readable)
+  def recv_nonblock: (Integer maxlen, ?Integer flags, ?String buf, ?exception: true) -> String?
+                   | (Integer maxlen, ?Integer flags, ?String buf, exception: false) -> (String | :wait_readable | nil)
 
   # <!--
   #   rdoc-file=ext/socket/lib/socket.rb
@@ -402,8 +402,8 @@ class BasicSocket < IO
   # recvmsg_nonblock should not raise an IO::WaitReadable exception, but return
   # the symbol <code>:wait_readable</code> instead.
   #
-  def recvmsg_nonblock: (?Integer dlen, ?Integer flags, ?Integer clen, ?exception: true, ?scm_rights: boolish) -> [ String, Addrinfo, Integer?, Array[Socket::AncillaryData] ]
-                      | (?Integer dlen, ?Integer flags, ?Integer clen, exception: false, ?scm_rights: boolish) -> ([ String, Addrinfo, Integer?, Array[Socket::AncillaryData] ] | :wait_readable)
+  def recvmsg_nonblock: (?Integer dlen, ?Integer flags, ?Integer clen, ?exception: true, ?scm_rights: boolish) -> [ String, Addrinfo, Integer?, Array[Socket::AncillaryData] ]?
+                      | (?Integer dlen, ?Integer flags, ?Integer clen, exception: false, ?scm_rights: boolish) -> ([ String, Addrinfo, Integer?, Array[Socket::AncillaryData] ] | :wait_readable | nil)
 
   # <!--
   #   rdoc-file=ext/socket/basicsocket.c

stdlib/socket/0/socket.rbs

@@ -1365,8 +1365,8 @@ class Socket < BasicSocket
   # ### See
   # *   Socket#recvfrom
   #
-  def recvfrom_nonblock: (Integer maxlen, ?Integer flags, ?untyped outbuf, ?exception: true) -> [ String, Addrinfo ]
-                       | (Integer maxlen, ?Integer flags, ?untyped outbuf, exception: false) -> ([ String, Addrinfo ] | :wait_readable)
+  def recvfrom_nonblock: (Integer maxlen, ?Integer flags, ?untyped outbuf, ?exception: true) -> [ String, Addrinfo ]?
+                       | (Integer maxlen, ?Integer flags, ?untyped outbuf, exception: false) -> ([ String, Addrinfo ] | :wait_readable | nil)
 
   # <!--
   #   rdoc-file=ext/socket/socket.c

stdlib/socket/0/udp_socket.rbs

@@ -86,8 +86,8 @@ class UDPSocket < IPSocket
   # ### See
   # *   Socket#recvfrom
   #
-  def recvfrom_nonblock: (Integer len, ?Integer flag, ?String outbuf, ?exception: true) -> [ String, [ String, Integer, String, String ] ]
-                       | (Integer len, ?Integer flag, ?String outbuf, exception: false) -> ([ String, [ String, Integer, String, String ] ] | :wait_readable)
+  def recvfrom_nonblock: (Integer len, ?Integer flag, ?String outbuf, ?exception: true) -> [ String, [ String, Integer, String, String ] ]?
+                       | (Integer len, ?Integer flag, ?String outbuf, exception: false) -> ([ String, [ String, Integer, String, String ] ] | :wait_readable | nil)
 
   # <!--
   #   rdoc-file=ext/socket/udpsocket.c