Attempt to document method grabpairs in role Baggy

gistfile1.txt

=head2 method grabpairs

Defined as:

    multi method grabpairs(Baggy:D:) returns Any
    multi method grabpairs(Baggy:D: $count) returns List:D

Returns a C<Pair> or a C<List> of C<Pair>s depending on the version of the method
being invoked. Each C<Pair> returned has an element of the invocant as its key and the
elements weight as its value. Unlike L<pickpairs>, it works only on mutable structures,
e.g. L<BagHash|/type/BagHash>. Use of C<grabpairs> on an immutable structure results in
an C<X::Immutable> exception. If C<*> is passed as C<$count>, or C<$count> is greater
than or equal to the number of L<elements|#method elems> of the invocant, then all
element/weight C<Pair>s from the invocant are returned in a random sequence.

What makes C<grabpairs> different from L<pickpairs|#method pickpairs> is that the
'grabbed' elements are in fact removed from the invocant. 

    my $breakfast = (eggs => 2, bacon => 3).BagHash;
    say $breakfast.grabpairs;                         # bacon => 3
    say $breakfast.grabpairs(1);                      # (eggs => 2)
    say $breakfast.grabpairs(*);                      # []