cgroups: update documentation about css_set hash table

When a task is moved from one cgroup to another, it gets a new

css_set pointer - if there's an already existing css_set with the

desired collection of cgroups then that group is reused, else a new

css_set is allocated. Note that the current implementation uses a

linear search to locate an appropriate existing css_set, so isn't

very efficient. A future version will use a hash table for better

performance.

css_set is allocated. The appropriate existing css_set is located by

looking into a hash table.

To allow access from a cgroup to the css_sets (and hence tasks)

that comprise it, a set of cg_cgroup_link objects form a lattice;