Add description of `saladVersion`

[tom-tan]

This request clarifies the existence of saladVersion, that is currently undocumented.

I'm afraid that added description may conflict with the existing description:

• Added:

  Each document must declare the saladVersion of that document

• Existing description that may conflict with added one:

  A Salad document must consist only of either a single root object or an array of objects.

Please let me know if there are better descriptions for it.

[codecov[bot]]

Codecov Report

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

Project coverage is 83.05%. Comparing base (19ac14b) to head (50107f0).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #861   +/-   ##
=======================================
  Coverage   83.05%   83.05%           
=======================================
  Files          22       22           
  Lines        4886     4886           
  Branches     1326     1326           
=======================================
  Hits         4058     4058           
  Misses        548      548           
  Partials      280      280           

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