Clarify first sentence of the CDM Attacks and Vulnerabilities section

[ddorwin]

A concern about the first sentence of the CDM Attacks and Vulnerabilities section was raised in #383 (comment). We should address that independent of the outcome of that issue. The sentence currently says:

User Agents are responsible for providing users with a secure way to browse the web, including any functionality, such as CDMs, from third parties.

In #383 (comment), I proposed the following replacement:

User Agents are responsible for providing users with a secure way to browse the web. This responsibility applies to any functionality used by User Agents, including functionalities from third parties.

Other suggestions welcome. The reference to "CDMs" is dropped to make the sentence easier to read; this is probably implied by the section, but it would be nice to find a way to include.

[mwatson2]

@ddorwin I think your proposal is a good one.

[dwsinger]

fine by me.

[jdsmith3000]

LGTM as well.