Reword and add explanatory comments for MAX_IM_CA tests
Reword and add explanatory comments for MAX_IM_CA tests

- Reword the comment on config.h to suggest that
  `MAX_INTERMEDIATE_CA` may not exist in the config.
- Add a comment explaining why the tests are more restrictive
  than necessary.

Signed-off-by: Yuto Takano <0d4e4f819ef3797eef849c0f9212ede105948585@arm.com>

Mbed-TLS/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls