core: add inline description of TEE session field user_ctx
core: add inline description of TEE session field user_ctx

Replace inline comment ??? with an appropriate description of
the user_ctx field which points the the caller user TA context
in case a pseudo TA is invoked from the user TA client.

Signed-off-by: Etienne Carriere <f21bc5b8f3ea1962eb45af359f1abea6578cab94@linaro.org>
Reviewed-by: Jens Wiklander <7706914404370d7502c27a0eff493dcf491feb51@linaro.org>

pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os