Document $~ before dependent global variables

doc/globals.rdoc

@@ -4,12 +4,12 @@
 
 $!::   The exception information message set by 'raise'.
 $@::   Array of backtrace of the last exception thrown.
+$~::   The information about the last match in the current scope (thread-local and frame-local).
 $&::   The string matched by the last successful match.
 $`::   The string to the left  of the last successful match.
 $'::   The string to the right of the last successful match.
 $+::   The highest group matched by the last successful match.
 $1::   The Nth group of the last successful match. May be > 1.
-$~::   The information about the last match in the current scope.
 $=::   This variable is no longer effective. Deprecated.
 $/::   The input record separator, newline by default. Aliased to $-0.
 $\::   The output record separator for the print and IO#write. Default is nil.