Added docs for Baggy.grab

doc/Type/Baggy.pod

@@ -13,7 +13,29 @@ L<Mixy>.
 
 =head2 method grab
 
-=comment TODO
+Defined as:
+
+    multi method grab(Baggy:D:) returns Any
+    multi method grab(Baggy:D: $count) returns Array:D
+
+Like L<pick|#method pick>, a C<grab> returns a random selection of elements, weighted
+by the values corresponding to each key. Unlike C<pick>, it works only on mutable
+structures, e.g. L<BagHash|/type/BagHash>. Use of C<grab> on an immutable structure results in an
+C<X::Immutable> exception. If C<*> is passed as C<$count>, or C<$count> is greater than
+or equal to the L<total|#method total> of the invocant, then C<total> elements from the
+invocant are returned in a random sequence.
+
+Grabbing decrements the grabbed key's weight by one (deleting the key
+when it reaches 0). By definition, the C<total> of the invocant also decreases by one, so the
+probabilities stay consistent through subsequent C<grab> operations.
+
+    my $cars = ('Ford' => 2, 'Rover' => 3).BagHash;
+    say $cars.grab;                                   # Ford
+    say $cars.grab(2);                                # [Rover Rover]
+    say $cars.grab(*);                                # [Rover Ford]
+
+    my $breakfast = ('eggs' => 2, 'bacon' => 3).Bag;
+    say $breakfast.grab;                              # throws X::Immutable exception
 
 =head2 method grabpairs