docs(ngCsp): update explanation of CSP rules and how they affect Angular

Update the description of CSP, mainly regarding `unsafe-eval` and `unsafe-inline`. The way it was
presented previously was slightly misleading as it indicated that these were rules forbidding
certain things, when in fact it's a keyword in the CSP that disables the very rule that was
described. The updated text clarifies this better.

Closes #15142