Clarify behaviour of `json` parameter.

`json` is ignored if `data` is not empty.

How about this?

> The `data` parameter takes precedence over the `json` parameter.

# [Codecov](https://codecov.io/gh/requests/requests/pull/4368?src=pr&el=h1) Report
> Merging [#4368](https://codecov.io/gh/requests/requests/pull/4368?src=pr&el=desc) into [master](https://codecov.io/gh/requests/requests/commit/81b63419cbd8a6f0c9c2b79083d25dc2260156e3?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/requests/requests/pull/4368/graphs/tree.svg?width=650&height=150&src=pr&token=mZGswIiXLa)](https://codecov.io/gh/requests/requests/pull/4368?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #4368   +/-   ##
=======================================
  Coverage   88.28%   88.28%           
=======================================
  Files          18       18           
  Lines        2031     2031           
=======================================
  Hits         1793     1793           
  Misses        238      238
```



------

[Continue to review full report at Codecov](https://codecov.io/gh/requests/requests/pull/4368?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/requests/requests/pull/4368?src=pr&el=footer). Last update [81b6341...b701555](https://codecov.io/gh/requests/requests/pull/4368?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


# [Codecov](https://codecov.io/gh/requests/requests/pull/4368?src=pr&el=h1) Report
> Merging [#4368](https://codecov.io/gh/requests/requests/pull/4368?src=pr&el=desc) into [master](https://codecov.io/gh/requests/requests/commit/acd2645444d280ac0c5d2d227fdd222cb1ac609c?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/requests/requests/pull/4368/graphs/tree.svg?height=150&width=650&token=mZGswIiXLa&src=pr)](https://codecov.io/gh/requests/requests/pull/4368?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #4368   +/-   ##
=======================================
  Coverage   89.05%   89.05%           
=======================================
  Files          18       18           
  Lines        2055     2055           
=======================================
  Hits         1830     1830           
  Misses        225      225
```



------

[Continue to review full report at Codecov](https://codecov.io/gh/requests/requests/pull/4368?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/requests/requests/pull/4368?src=pr&el=footer). Last update [acd2645...39446de](https://codecov.io/gh/requests/requests/pull/4368?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


@nateprewitt Thanks for the comments and apologies for the delay. I've updated as suggested.

Yep, looks great to me too, thanks @danielroseman!