Document break tag theme rendering behavior

[NathanPJF]

This updates the Liquid break tag documentation to match its behavior in both iteration and theme rendering contexts:

• Clarifies that break stops a for/tablerow loop when used inside a loop.
• Documents that break stops rendering of the current file when used outside a loop, while preserving previously rendered output.
• Splits the examples into "Iteration usage" and a new "Theme rendering usage" section.

This is a documentation-only change; runtime behavior is unchanged.

Follow-up work

Public docs are being updated in a separate PR