Better document the interaction of SsrModes with blocking resources

[g2p]

Meant to address users making the same mistake as
#1119

[g2p]

By the way, I wonder if PartiallyBlocked might become the default. I don't see the point of having the expected HTML seemingly ready but hidden within an unrendered template.

[gbj]

Thanks! This is really helpful and looks accurate to me. PartiallyBlocked was added after this chapter was written, and I clearly didn't do a good job of explaining it. I appreciate the help.

[gbj]

By the way, I wonder if PartiallyBlocked might become the default. I don't see the point of having the expected HTML seemingly ready but hidden within an unrendered template.

It's purely a performance thing: PartiallyBlocked is really only useful in situations that do not support any JavaScript usage, including inline script tags, at the cost of an extra O(n) string replace on the server. I guess I'm open to argument in the other direction, but if you want to have that conversation we should do it in a separate discussion or issue.