docs: Clarify nesting of context events

[ueno]

Also add pk::rsa_padding and tls::verify_cert_chain.

[neverpanic]

Some nitpicks, but in general I like the direction this is going.

[neverpanic]

Suggested change

	A protocol is typically defined in a context hierarchy. In this
	document they are defined in the following presentation syntax.
	Tracing information for most protocols can be represented in a
	hierarchical structure of contexts and events. This document
	defines the presentation syntax for this tree.

I feel this describes better what it is we're trying to do here, let me know what you think.

[neverpanic]

Should we add explanations what the various fields mean in which contexts, and what string representations we expect? Or should we leave that up to the implementations to define and then we'll have to process whatever we get from the libraries in our monitoring tooling?

If we don't define anything, we'll probably get "RSASSA-PSS", "RSA_PSS", and "PSS" all for the same padding scheme, and the same applies for algorithm, curve, and hash, too.

I also feel we may want to make it more explicit that all fields are optional, i.e., an RSA sign event might omit the curve event, and an ECDSA signature can come without rsa_padding.

[neverpanic]

The examples above don't use group. Should the derive event use group instead of curve?