Clarify cgroups introduction

Explain the term "partition".

Delete the phrase "set of parameters" which obfuscates the
definition of cgroup.

Crisp up the definition of subsystem.

Describe the representation of the cgroup virtual filesystem
since this is not specifically described later in the document.

Each hierarchy must be associated with at least one subsystem.

One or more given subsystems may be attached to multiple
hierarchies, although this isn't very useful.

Use the term "task" in preference to "process" everywhere,
for consistency.

Interested readers may wish to read the formal model of control groups
of which this commit is a spin-off:

https://github.com/Zteve/container-specs/raw/master/cgroups/cgroups.pdf