DOC: config: restore default values for resolvers hold directive

DOC: config: restore default values for resolvers hold directive

Default values for hold directive (resolver context) used to be documented
but this was lost when the keyword description was reworked in 24b319b
("Default value is 10s for "valid", 0s for "obsolete" and 30s for
others.")

Restoring the part that describes the default value.

It may be backported to all stable versions with 24b319b