Update PTY.open documentation to document it yields a single argument [ci skip]

For a regular block, accepting two arguments is fine as the array
will be autosplatted.   However, a lambda that accepts two arguments
will not work.

We could change the implementation to yield two arguments instead
of an array with a single argument, but that would be less backwards
compatible.

I'm only changing the call-seq to be precise, other examples pass
a literal block that accepts two arguments, and I left those alone
as that will be the most common usage.

Fixes [Bug #17094]

ext/pty/pty.c

@@ -464,11 +464,11 @@ pty_close_pty(VALUE assoc)
 /*
  * call-seq:
  *   PTY.open => [master_io, slave_file]
- *   PTY.open {|master_io, slave_file| ... } => block value
+ *   PTY.open {|(master_io, slave_file)| ... } => block value
  *
  * Allocates a pty (pseudo-terminal).
  *
- * In the block form, yields two arguments <tt>master_io, slave_file</tt>
+ * In the block form, yields an array of two elements (<tt>master_io, slave_file</tt>)
  * and the value of the block is returned from +open+.
  *
  * The IO and File are both closed after the block completes if they haven't