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ARROW-6660: [Rust] [DataFusion] Minor docs update for 0.15.0 release

Change the top-level description of DataFusion to better reflect what it actually does today.

Also added a little more detail to some other docs.

Closes #5466 from andygrove/datafusion-docs-update and squashes the following commits:

c5df6e14d <Andy Grove> Minor docs update for 0.15.0 release

Authored-by: Andy Grove <andygrove73@gmail.com>
Signed-off-by: Paddy Horan <paddyhoran@hotmail.com>