Remove some requirements of SXG payload

[caoboxiao]

Update the document and remove the requirement of Google AMP cache
SXG payload to be unchanged after transform.

[twifkak]

Hi @caoboxiao! Might as well also remove

amppackager/docs/cache_requirements.md

Lines 23 to 25 in 15fd6ec

	* well-formed UTF-8 that doesn't contain:
	* any characters that cause a parse-error during [HTML preprocessing](https://html.spec.whatwg.org/multipage/parsing.html#preprocessing-the-input-stream)
	* U+0000 NULL

since that is also obviated by http://cl/320958457.

[caoboxiao]

Done.
The utf-8 related requirement of SXG payload is no longer needed.

Hi @caoboxiao! Might as well also remove

amppackager/docs/cache_requirements.md

Lines 23 to 25 in 15fd6ec

	* well-formed UTF-8 that doesn't contain:
	* any characters that cause a parse-error during [HTML preprocessing](https://html.spec.whatwg.org/multipage/parsing.html#preprocessing-the-input-stream)
	* U+0000 NULL

since that is also obviated by http://cl/320958457.

[twifkak]

I sent you an invite for write access on this repo. As soon as you accept, you can squash and merge (per our policy).