docs: add inline cases condition in no-fallthrough

[Tanujkanti4441]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request? (put an "X" next to an item)

[x] Documentation update
[ ] Bug fix (template)
[ ] New rule (template)
[ ] Changes an existing rule (template)
[ ] Add autofix to a rule
[ ] Add a CLI option
[ ] Add something to the core
[ ] Other, please explain:

What changes did you make? (Give an overview)

no-fallthrough rule docs says

By default, this rule does not require a fallthrough comment after an empty case only if the empty case and the next case are on the same line or on consecutive lines.

docs already has an example with consecutive lines and this PR adds the example with the same line condition.

Is there anything you'd like reviewers to focus on?

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[amareshsm]

LGTM. Leaving this open for the 2nd review.

[nzakas]

@amareshsm please remember to move PRs like this to "Implementing" on the triage board.

[nzakas]

LGTM. Thanks!