Add notes $?USAGE was renamed to $*USAGE

Rakudo impl: rakudo/rakudo@0b15f6728a
Spec: Raku/roast@04b03b1d40
Docs: Raku/doc@9849f0945f

S06-routines.pod

@@ -3413,7 +3413,8 @@ usage message is printed to standard output instead.
 
 This usage message is automatically generated from the signature (or
 signatures) of C<MAIN>.  This message is generated at compile time,
-and hence is available at any later time as C<$?USAGE>.
+and hence is available at any later time as C<$?USAGE> (EDIT: variable
+renamed to C<$*USAGE> and message is generated on-demand, at runtime).
 
 Common Unix command-line conventions are mapped onto the capture
 as follows:

S28-special-names.pod

@@ -117,7 +117,7 @@ be fleshed out in S26.
  $?SCOPE           S02                # Current "my" scope (XXX unnecessary?)
  $*TMPDIR          S16   IO::Path     # system temporary directory
  $*TZ              S32                # Local time zone
- $?USAGE           S06   Str          # Default usage message generated at compile time
+ $?USAGE           S06   Str          # Default usage message generated at compile time (EDIT: was renamed to $*USAGE)
  $*USER                  +Int,~Str    # user id (numeric) or name (string)
  $?VM              S02   Systemic     # Which virtual machine am I compiling under
  $?XVM             S02   Systemic     # Which virtual machine am I cross-compiling for