docs(concat): clarify array inputs

[beanscg]

Fixes #4697.

This updates the concat documentation to remove the misleading statement that an array of Observables can be passed directly. Since concat accepts rest arguments, passing an array emits the Observable objects in that array; callers that already have an array should spread it with concat(...timers).

Changes:

• Clarify that each Observable should be passed as its own argument.
• Explain the array-spread behavior and the failure mode of concat(timers).
• Add an array-of-Observables example using concat(...timers).

Validation:

• git diff --check
• Node content check for the updated warning and concat(...timers) example

Full docs/test suite was not run locally because this environment does not have this repository's dependencies installed.