cmDocumentation: `char*[][2]` → `cmDocumentationEntry[N]`

Use fixed size arrays of `cmDocumentationEntry` items instead of
open arrays of two `char` pointers when describe program options
help screens.

Also, drop `const char*[][2]` overloads of methods of
`cmDocumentation` and `cmDocumentationSection` classes in the sake
of generic (template) appenders introduced earlier.