Negative binary shifts are indeed tested in roast, so whirl them

into specs.  This implies that :signed and :unsigned will be
useful on infix:«<+», so mention that.  (The :signed and :unsigned
adverbs are currently only in spec not in roast/implementation)

S03-operators.pod

@@ -870,6 +870,9 @@ C<< infix:«+<» >>, numeric shift left
 
     $integer +< $bits
 
+Shifts by a negative number of bits cause a corresponding right shift.
+As such the C<:signed> or C<:!signed> adverbs may be applied (see below.)
+
 =item *
 
 C<< infix:«+>» >>, numeric shift right
@@ -879,6 +882,8 @@ C<< infix:«+>» >>, numeric shift right
 By default, signed types do sign extension, while unsigned types do not, but
 this may be enabled or disabled with a C<:signed> or C<:!signed> adverb.
 
+Shifts by a negative number of bits cause a corresponding left shift.
+
 =item *
 
 C<< infix:<~&> >>, buffer bitwise and