YJIT: Edit module documentation for clarity

Add an empty line before the module doc string so RDoc can find it.
While we are at it, edit for clarity. The file should already be
using frozen string literals since c10d5085a2.

[ci skip]

yjit.rb

@@ -1,10 +1,13 @@
-# This module allows for introspection on YJIT, CRuby's experimental in-process
-# just-in-time compiler. This module is for development purposes only;
-# everything in this module is highly implementation specific and comes with no
+# frozen_string_literal: true
+
+# This module allows for introspection of YJIT, CRuby's experimental in-process
+# just-in-time compiler. This module exists only to help develop YJIT, as such,
+# everything in the module is highly implementation specific and comes with no
 # API stability guarantee whatsoever.
 #
-# This module is only defined when YJIT has support for the particular platform
-# on which CRuby is built.
+# This module may not exist if YJIT does not support the particular platform
+# for which CRuby is built. There is also no API stability guarantee as to in
+# what situations this module is defined.
 module RubyVM::YJIT
   if defined?(Disasm)
     def self.disasm(iseq, tty: $stdout && $stdout.tty?)