chore(documentation): Add exists documentation

[rolljee]

What does this PR do ?

• This PR adds the documentation that was missing from the exists method inside this sdk

[codecov[bot]]

Codecov Report

Merging #729 (958ca25) into 7-dev (d9660d3) will not change coverage. The diff coverage is n/a.

:exclamation: Current head 958ca25 differs from pull request most recent head 66c1ae2. Consider uploading reports for the commit 66c1ae2 to get more accurate results

@@           Coverage Diff           @@
##            7-dev     #729   +/-   ##
=======================================
  Coverage   83.04%   83.04%           
=======================================
  Files          46       46           
  Lines        2112     2112           
  Branches      408      408           
=======================================
  Hits         1754     1754           
  Misses        279      279           
  Partials       79       79           

[kuzzle]

:tada: This PR is included in version 7.11.0-beta.1 :tada:

The release is available on:

• npm package (@beta dist-tag)
• GitHub release

Your semantic-release bot :package::rocket:

[kuzzle]

:tada: This PR is included in version 7.11.0 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot :package::rocket: