[SPARK-39982][DOC] Add doc string to StructType.fromJson

[khalidmammadov]

What changes were proposed in this pull request?

Documentation provided for StructType.fromJson method

Why are the changes needed?

To make it easy for a user to understand this method and use it. It was inspired by this SO question

Does this PR introduce any user-facing change?

Yes, adding missing documentation

How was this patch tested?

Unit tests

[AmplabJenkins]

Can one of the admins verify this patch?

[srowen]

Minor changes requested above, but if you can make the tweaks, I think this is OK

[khalidmammadov]

Additionally I have checked how it looks using bundle exec jekyll serve --host 0.0.0.0

[srowen]

Merged to master