Clarify cgroups introduction

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

Crisp up the definition of subsystem.

Explain the term "partition".

Clarify that each hierarchy must be associated with at least one
subsystem.

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

Clarify that subsystems may be attached to multiple hierarchies,
although this isn't very useful, and explain what happens.

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

The following formal model of control groups helped in producing
the above clarifications:

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