[Doc] Better document the Tensor classes in the prose documentation

From a comment from @wesm in ARROW-2714:

> The Tensor classes are independent from the columnar data structures, though they reuse pieces of metadata, metadata serialization, memory management, and IPC.
>
> The purpose of adding these to the library was to have in-memory data structures for handling Tensor/ndarray data and metadata that "plug in" to the rest of the Arrow C++ system (Plasma store, IO subsystem, memory pools, buffers, etc.).
>
> Theoretically you could return a Tensor when creating a non-contiguous slice of an Array; in light of the above, I don't think that would be intuitive.
>
> When we started the project, our focus was creating an open standard for in-memory columnar data, a hitherto unsolved problem. The project's scope has expanded into peripheral problems in the same domain in the meantime (with the mantra of creating interoperable components, a use-what-you-need development platform for system developers). I think this aspect of the project could be better documented / advertised, since the project's initial focus on the columnar standard has given some the mistaken impression that we are not interested in any work outside of that.

**Reporter**: [Joris Van den Bossche](https://issues.apache.org/jira/browse/ARROW-5858) / @jorisvandenbossche

_{**Note**: *This issue was originally created as [ARROW-5858](https://issues.apache.org/jira/browse/ARROW-5858). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}