net: ena: fix RST format in ENA documentation file

The documentation file used to be written in markdown format but was
converted to reStructuredText (rst).

The converted file doesn't keep up with rst format requirements which
results in hard-to-read text.

This patch fixes the formatting of the file. The patch also
* Highlights and emphasizes some lines to improve readability
* Rephrases some hard-to-understand text
* Updates outdated function descriptions.
* Removes TSO description which falsely claims the driver supports it

Signed-off-by: Shay Agroskin <shayagr@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>