* Update description of certificate fetch API
Signed-off-by: Alexander Scheel <alex.scheel@hashicorp.com>

* Clarify /config/crl and /config/url PKI are empty

GET-ing these URLs will return 404 until such time as a config is posted
to them, even though (in the case of CRL), default values will be used.
Signed-off-by: Alexander Scheel <alex.scheel@hashicorp.com>

* Clarify usage of /pki/crl/rotate
Signed-off-by: Alexander Scheel <alex.scheel@hashicorp.com>

* Update documentation around PKI key_bits

This unifies the description of key_bits to match the API description
(which is consistent across all usages).
Signed-off-by: Alexander Scheel <alex.scheel@hashicorp.com>

* Fix indented field descriptions in PKI paths
Signed-off-by: Alexander Scheel <alex.scheel@hashicorp.com>

* Clarify documentation around serial_number

Note that this field has no impact on the actual Serial Number field and
only an attribute in the requested certifi...