Indicate explicitly the intent of the Web Share API regarding cross-origin

[siusin]

Section 4 indicates that, by default, the API is not allowed to be used in cross-origin contexts. It says so in the sentence "Its default allowlist is 'self'.". Given the incomplete state/lack of implementations for Permissions Policy, section 4 should indicate explicitly the intent of the Web Share API regarding cross-origin, independently of Permissions Policy. It can do so by adding a third sentence to the first paragraph of section 4, either by copying the definition of self or paraphrasing the intent. In addition, an editorial note will need to be added to that section to alert developers on the current lack of implementation experience from Permission Policy.

/cc @plehegar

[siusin]

Addressed by #256.

Thanks!