Minor cleanups to GLFM update_example_snapshots.rb script and spec

What does this MR do and why?

Some minor cleanups to GLFM scripts/lib/glfm/update_example_snapshots.rb script and spec:

1. Addresses suggestion from !95437 (comment 1069782684)
2. Adds missing h3 hierarchy number in comment which was missed in Add support for nesting GLFM examples in H3s (!95302 - merged), and also moves and expands the text of the comment with a TODO to make its purpose more clear
3. Adds a clarifying nesting describe block for the section of update_example_snapshots_spec.rb which tests the fast-testable behavior of glfm_example_status.yml.

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