docs: Add some clarifications to JWT docs
docs: Add some clarifications to JWT docs

This adds some highlights to sections that commonly trip up users and
gives some additional examples for how the functions work and can
interact.

Fixes: #1916
Signed-off-by: Patrick East <1a015b2c287ee8e96f2ec59ce4953b89b37de4a8@gmail.com>

tsandall/opa,tsandall/opa,tsandall/opa,open-policy-agent/opa,open-policy-agent/opa,open-policy-agent/opa,open-policy-agent/opa,tsandall/opa,open-policy-agent/opa,tsandall/opa,open-policy-agent/opa,tsandall/opa