doc: update http.md for consistency and clarity

The HTTP Keep-Alive text was inconsistent. These changes follow the
following rules

* When referring to flag used in code, it should always be written using
  backticks and in camel case. E.g. `keepAlive`.
* When referring to the mechanism Keep-Alive functionality as described
  in the HTTP 1.1 RFC, it is written as 'HTTP Keep-Alive', without the
  use of backticks.
* When referring to the request header, it should always use backticks
  and be written as `Connection: keep-alive`.

This commit also includes some changes to how `http.Agent` is
referenced. When `Agent` is used as a reference to an object or
instance, backticks should always be used.

And lastly, the documentation about `Agent` behavior around HTTP
Keep-Alive has been clarified and improved.

Fixes: https://github.com/nodejs/node/issues/10567
PR-URL: https://github.com/nodejs/node/pull/10715



Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>