tor-spec: Revise the CircID selection section

This commit updates the "5.1.1. Choosing circuit IDs in create cells"
section, in order to clarify its importance, as well as to adjust it to
modern link protocol versions.

The first goal is achieved, by directly adding a "MUST" in the first
paragraph, alongside a reformulation in the paragraph explaining the
method in link protocol version 4 or higher.

The second goal is achieved by merging the second paragraph with the
third paragraph, as the second paragraph only applies to the link
protocol versions addressed in the third one.

0 files changed, 0 insertions, 0 deletions