Help: Expand documentation of CMAKE_VERSION and related variables

Describe the meaning of each version component in more detail in the
documentation of CMAKE_VERSION.  Simplify the per-component version
variable documentation by referencing the main variable.

Include information about how to compare version strings.  Also add
an historical note about the version scheme used prior to commit
v2.8.2~105^2~4 (New version scheme to support branchy workflow,
2010-04-23).
Merge topic 'doc-version-components' into next

52e7beb6 Help: Expand documentation of CMAKE_VERSION and related variables