Minor updates on #180

[iherman]

• took care of https://github.com/w3c/pub-manifest/pull/180#discussion_r357962658 and https://github.com/w3c/pub-manifest/pull/180#discussion_r357962970
• to handle the issue https://github.com/w3c/pub-manifest/pull/180#issuecomment-567124310 I exchanged the word 'exit' in the relevant sentences to 'omit', with a further clarification in the initial note

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Preview | Diff

[mattgarrish]

Will "omit the element" potentially cause confusion, since some information from the element is extracted?

What if we phrased it like "Skip further processing of the element and continue to the next."?

[iherman]

@mattgarrish, I have committed a new version changing the text to what you proposed in https://github.com/w3c/pub-manifest/pull/182#issuecomment-571715526