Closes #6328. Document 'json' selector type

[kumar-sanchay]

added json as one of the value for type argument. Closes #6328

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 88.45%. Comparing base (d7da298) to head (afaa256).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #6334 +/- ## ========================================== + Coverage 83.88% 88.45% +4.57% ========================================== Files 161 161 Lines 12010 12010 Branches 1929 1867 -62 ========================================== + Hits 10075 10624 +549 + Misses 1598 1030 -568 - Partials 337 356 +19 ``` | [Files](https://app.codecov.io/gh/scrapy/scrapy/pull/6334?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scrapy) | Coverage Δ | | |---|---|---| | [scrapy/selector/unified.py](https://app.codecov.io/gh/scrapy/scrapy/pull/6334?src=pr&el=tree&filepath=scrapy%2Fselector%2Funified.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scrapy#diff-c2NyYXB5L3NlbGVjdG9yL3VuaWZpZWQucHk=) | `100.00% <ø> (ø)` | | ... and [16 files with indirect coverage changes](https://app.codecov.io/gh/scrapy/scrapy/pull/6334/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scrapy)

[Gallaecio]

Thanks!