shorter than B<n> bits.
BN_lshift() shifts B<a> left by B<n> bits and places the result in
-B<r> (C<r=a*2^n>). BN_lshift1() shifts B<a> left by one and places
-the result in B<r> (C<r=2*a>).
+B<r> (C<r=a*2^n>). Note that B<n> must be non-negative. BN_lshift1() shifts
+B<a> left by one and places the result in B<r> (C<r=2*a>).
BN_rshift() shifts B<a> right by B<n> bits and places the result in
-B<r> (C<r=a/2^n>). BN_rshift1() shifts B<a> right by one and places
-the result in B<r> (C<r=a/2>).
+B<r> (C<r=a/2^n>). Note that B<n> must be non-negative. BN_rshift1() shifts
+B<a> right by one and places the result in B<r> (C<r=a/2>).
For the shift functions, B<r> and B<a> may be the same variable.